Well, an update from my tests. On my 4G wifi box, which I've now told my N10 is a hotspot device, it seems to be behaving itself.

This was from a 70 minute session and as you can see it didn't use an unusual amount of data.
My own laptop computer only used some 30MB or so in a 3 hour session.
Last night I tried it on the wifi at the library for some 2 hours. It used close to 100MB, but 30MB of that was app updates.
The rest of it was browsing in Google Play Store for books, music, reading magazine articles in the Newsstand, etc.
The Google Play Store really eats up data, so does Google Plus and Facebook, largely from all the images they load.
I did not play any music samples or stream any music, YouTube, and so forth.
I've got notifications turned off now except for Gmail, Email, and Google Play Store, and no widgets like "Recommended for You" which I suspect eats up a lot of data all by itself.
So... maybe it had to download some updates here and there that it didn't tell me about. Past major jumps in usage were for big things like the recent KitKat update.