Does iOS report usage in megabytes or mebibytes?
I wasn't able to find conclusive documentation on this so I tested it. I found that iOS uses the base-10 definition when showing usage. 

That is, if you send a large file, say 10 mebibytes (10 × 220, or 10485760 bytes), it will report it in megabytes, counting a megabyte as 1,000,000 bytes (not 1,048,576 bytes).

So that 10 MB, shows up as 10.4 MB:



(I'm not suggesting that there's anything nefarious going on here related to billing or data limits.)