How Many Pixels for 20 X 30 Print?

[Fleetwood271]

I took a picture on Sat and was just on a site uploading it and about to order a 20 X 30 print. However, the site said the resolution was not sufficient for a 20 X 30 print.
I checked my picture and it is 4127 X 2692 pixels. The site has a chart that shows what resolution they need for each size print. Here is what confused me: Their chart said for a 20 X 30 they need 2700 X 1800.
My picture is much larger than that. Why are they refusing my picture?

One thought that I've had while typing this is that the resolution changed when I uploaded it. Could that be the problem? The picture is large enough on my computer, but on their site it has been re-sized?