[Q] problem with my phones

my phone and tab (phone is karbonn A15 and Tab is Micromax Funbook P300) they both have the same problem. My tab is rooted and the phone is not. the problem is, that both companies promise me 512 mb ram but in karbonn it is showing 396 mb ram, and in the tab it shows correct- 512 mb, but when i flash a rom, it reverts to 396 mb.

P.S= My tab came rooted from the company and i didn't root it.

Thanks for your help..