Why MBP 13' has only 1280x800 res. when the MBA with the SAME graphic card has 1440x900? I mean, both use Intel HD Graphics 3000, and the MBP actually has a better processor. If they use the same Intel graphic card (and according to the website, the same info "13.3-inch LED-backlit glossy widescreen display", so they don't say they are different), how come MBP can't have 1440x900? What is the technical limitation of Apple hardware in MBP that justify this difference?
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.