I will agree with you.There are loopholes in calculating TRP .
Even I feel The TRPs do not give correct picture of the viewers of the show.
Only the viewers of Big cities are considered what about the viewers from small cities and villages?and what about viewers from abroad.?
The most watched serial gets more points & the less watched one gets less points.
This is how the TRPs are calculated
If you want some more information go to this site.
http://en.wikipedia.org/wiki/Television_Rating_Point