Dissappearing Data

Used the Vodacom menu to obtain detailed balances on my remaining data in my data bundle. At 8:39am I had 1.99GB remaining until 30/11/2015. At 22:28 I did this again and this time I had 0GB remaining. It can be difficult to estimate usage but I didn't use much throughout the day, no movie downloads etc. I then check my data usage for the day in my cell settings and it says 170MB. Perhaps it measures usage slightly differently but more than 10 times more??