I have taken about a thousand photos at that time. Pretty much all of them are sharp and clear. Few of these are blurry. Is this normal, or my dslr is not 100% reliable.
Tv and Av are shooting modes not camera settings themselves.
So correctly speaking you shot at :
Shutter speed 1/100sec
Aperture - f4.5
Now as for the softness its possible from several sources:
1) Handshake. If you're standing correctly with good shooting posture then you should be able to get a minimum shutter speed of 1/focal length of the lens. So at 1/100sec you should be fine for sharp shot with a 100mm or shorter focal length lens (as a very rough rule of thumb - fatigue and poor posture will reduce that focal length whilst good posture, health and such can potentially increase it a little).
This kind of blur will appear as an even amount over the whole scene
2) Subject motions - this will appear most where the subject is moving the most (ergo you don't get a uniform amount of blur over the whole shot). This is easily possible at slower shutter speeds and at 1/100sec its very possible that this is the cause of the blurr in this photo
3) Missed focus - if you miss the focus you can potentially end up with the key subject being out of focus - thus appearing blurry.
4) Thin depth of field - this won't cause blur as such, but it will result in a very thin depth of field that can make the majority of the subject appear softer - note that if you combine this with the 3rd item you can get even worse results.
NOTE whilst flash can freeze motion in order to do this the flash light must be dominant in the scene (ergo if the shot were taken without the flash the subject would be fully underexposed- black). If flash is just giving fill lighting, then the ambient light will be dominant and as such you are back to the above mentioned situations where blur can be recorded and the flash won't prevent it.
OP image seems clearly to me to be hand shake. The tiles in the background are just as blurry as the subjects, so I don't think it's them moving. Also, they don't look like they were in a very active pose.
Once in a hundred images or whatever, your hand will just shake especially violently. Bell curves are like that.
I agree this looks like a bit of camera shake. Your EXIF data says you were zoomed to 34mm -- normally 1/100th is more than fast enough for that focal length even without any form of image stabilization -- but those guidelines are based on an assumption that you're at least trying to be somewhat steady and that you've got good technique for that.
It's also possible that it just missed focus. Off the top of my head, three things can cause this.
1) If the camera gets to select it's focus point and there's a distracting element which is CLOSER to you then your intended subject, the AF system will always try to pick the NEAREST object where it can lock focus. This isn't the issue with this particular shot though because we'd see that "nearer" object. If there is a possibility of distracting elements ruining the shot by causing the camera to focus on the wrong thing, then pick your own focus point rather than letting the camera pick it.
2) If you are in "One Shot" mode then two things happen: (a) the camera will not take the shot until it can confirm that it has locked focus (that's called "focus priority") and (b) once the camera does lock focus, it shuts down the focus system. That means if either you move or your subject moves after the focus locked, it will NOT re-focus the camera... the focus distance will be left at whatever it was originally locked at. You can release and re-press the shutter to get it to re-evaluate. You can also configure an alternate back-button to demand that it re-focus whenever you press that button (very popular among sports photographers.) It's unlikely this was the problem in this case unless you locked focus and moved substantially.
3) If you are in "AI Servo" mode then the camera switches to "release priority" mode (which is the opposite of "focus priority" mode.) In this mode the camera WILL take the shot as soon as you FULLY press the shutter button and it will do so whether it was finished focusing the lens or not. Basically "release priority" means when YOU press the camera shutter release button, it WILL release and snap that shot even if the camera wasn't really ready. This mode is popular for sports photographers because YOU (the photographer) know what you intend to shoot and the camera doesn't. If a distracting element gets in the way and you know the camera is focused for your purposes then you don't want it second guessing you, blowing the "decisive moment" when you needed it to fire and refocusing to some wrong/distracting element in the frame. You'd be using impolite and colorful language with your camera if it did that.