Dude this is why they say that they give u speeds between 64 and 128kbps and not 128kbps exact.
after i got my connection a week or two after it i started experiencing the same prob i.e speeds between 80-100 only. I complained and the CC ppl told me what i've written above, hence i cud say nothing more. This is the evry reason why its said that only mtnl gives "True" 256kbps BB.
But overall i think airtel is ok, not that bad, coz in 780p.m ur getting about 13Kbps dwld rate with unltd dwld plus 24-29kBps dwld rate at night. while in mtnl u get 30Kbps dwld rate at 670p.m and only at night u get unltd. So its upto u if u wanna change or not.