Why MBP 13' has only 1280x800 res. when the MBA with the SAME graphic card has 1440x900?
The simple answer is that the MBP doesn't have a screen that can display 1440x900 - it's native resolution is 1280x800 (ie. it only has that many pixels). This is a physical limitation, not a software limitation. There is no point in sending a higher resolution image to the screen, since it can't display any higher detail.

I can't tell you why Apple chose a higher resolution display for the 13" MBA than the 13" MBP, given that I wasn't party to those decisions. The reasoning may be purely historical, however. The white MacBooks had a resolution of 1280x800. Apple introduced an aluminum unibody version of the MacBook with the same display in 2008. By mid-2009, Apple decided to rebrand the unibody MacBook as the 13" MacBook Pro - still keeping the 1280x800 display. My guess is that they just haven't gotten around to changing it yet.