AC1750 traffic meter suddenly changed past traffic volume

Yesterday the traffic meter said I've used around 80gigs of data and when I check today it has changed to 4gigs. It has also changed last month's traffic volume by around 200ish gigs. Any idea whats going on? Haven't changed any settings.

AC 1750 R6700v3  

Firmware Version
V1.0.4.98_10.0.71