I also want to mention that 400x240 is the correct aspect ratio for the tablet's screen (1.66 or 16:10) , so it fills the whole screen and looks really good.

640x352 is 1.8 aspect ratio (close to widescreen cinema) not the tablet's aspect ratio, and so it fills less of the screen. (640 * 1.25 = 800, 352 * 1.25 = 440, not 480).

I'd be interested to see you compare apples to apples here. Please crop the width of your video from 640 to 576 and re-encode. It is still not perfect, because it is hard to find numbers that "fit" the tablet's aspect ratio in that range and also meet the "must be divisible by 16" rule that many mp4 players most mp4 encoders require... (EDIT: thanks sjgadsby!)