Actually, you only
thought you saw
Your 50D's sensor is 4752 pixels wide and your screen is 640 pixels wide. 4752/640 will give about 7.4x "zoom" for a perceived 100% crop.
The LCD is 640 x 480 (VGA spec) and that gives 307,200 pixels derived from the 920,000 "dots" (three colours required on the LCD). The maths won't work on the height because the sensor has an aspect ratio of 1.5 but the LCD is 1.33 so the sensor image doesn't fill the height of the LCD without zooming.
Hope that makes sense
Bob
Edit...or maybe I've got it wrong :shrug: