Airtel unlimited 2g internet plan clarification

hi friends,
can anyone explain the difference between airtel 2g internet plan of tamil nadu
1.rs.225/- (2gb) unlimited after that throttled to reduced speed.
2.rs.243/-(2gb) unlimited internet
both 28 days validity!!!

i dont know what is the diff between the usage of net? is it only 2gb after that speed reduce?
then why the diff in amount?

am waiting to recharge…