Actinia there are times when semi-auto modes are too jumpy. Remember that the camera reads reflected light off a subject not direct light from the source. So a black surface and a white surface will reflect different amounts; furthermore the camera is always going for that middle grey value.
Thus you could be shooting in light that isn't changing, but with different coloured subjects filling a primary part of the metered segment of the frame. You might want the exposure itself the same for each shot, but the camera would want different exposures for each one. .
You'd find this a lot if you used an external light meter and read light from the source as opposed to reflected.