Airtel(799 Plan) getting 1MBPS speed

Im from chennai
i changed to airtel 2 months ago (From BSNL bcoz of very frustating CC)
My plan amount is Rs 799
I dont know the name of my Plan :P
its like
512KBps till 10GB
and 256KBPS after that and no free calls
But i used more than 10GB both the months and i saw no reduction in speeds
and suddenly this Month
WHOA speedtest.net/result/924583695.png
i just got a speed jump to more than 1Mbps and it has been the same for 2 weeks now,is it a new offer

Airtel is doubling the speed at same amount for all the plans, as confirmed by many users, that why you are getting 1mbps, but after 10Gb you will be shifted to 256kbps