charging more than the usage

Location/place: anakapalle

Name of company/service: BSNL

sir,
This is mahesh.i have bsnl broadband connection to my pc.i used to check my status of usage on my pc(by clicking status on broadband connection icon).but, when i check the usage in bsnl selfcare portal it is showing more units of kb than usage. i am using bandwidth meter also. the bsnl is charging excess out of my usage . i have two snap screen shots to verify this.
my broadband plan details are as follows
plan : combo 500
free usage : 1.5 gb
please provide me justice .
thanking you