Telcos here deliberately do that, hopefully u'll exceed the free data limit and pay additional charges.
i have the same problem on my ip5. Its right after i updated to ios 8.4 (July period) then the data spiked a lot. 7gb and 11gb for the past 2 mths. Googled a bit and saw that it's an ios software issue. I also reset my cellular data stats and tested the data usage. Surfed net on safari for 1 min took 80mb! Ridiculous.
Tried to restore phone but also same problem. Need to wait till ios9 to release and install, hopefully can solve this bug. In the meantime, call ur telco to waive the excess data charges cos i did.