I found a more detailed breakdown, which only increases my suspicions.
Note the blocked ad count per app, and that it's sorted by most ads to the top, and #3 is a mere 5. Also I pay for Pandora, so it doesn't have any ads. So where did it block those other 867k ads? Hmmm.
I'm assuming for the data section, the uploaded/downloaded numbers are after the blocked data is removed, so it's claiming I've used 15GB. I doubt it, but even if we believe them, ads are an order of magnitude more data than desired data?! Even considering I mostly read forums and news articles and I don't stream video, that doesn't seem reasonable.
I'm going to show this to the data analytics side of my team and see what they think of it.