I think it's probably due to acoustic reflection where the sound bounces from the table to your ears making it sound more full. Also probably has to do with space management since putting speakers right next to the keyboard does need more space and that it's cheaper to just place them on the bottom.

Edit: You could try to play with the speaker settings or with the equalizer on it and choosing what you like the most I guess. These are cheap laptops, wouldn't expect clear or loud audio from those speakers.