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Has anyone seen this? I pointed a Bookbub add to my website. Bookbub reported 250 clicks, my website reported 160. Huh?

I'm mostly speculating from my knowledge, not experience, but there may be many aspects in play - if the page fails to load (or the person closes it before it does), chances are it will count on BB nut not on your web.
Similarly, if the person uses some add-on or setting in the browser that prevents your web from seeing their visit (or where it came from), it may not register properly. Things like VPN, adblockers, and such can tamper with the process needed to register things.

Right. Then there is the not so generous interpretation, which I have not heard raised against Bookbub but certainly have heard raised against Facebook. That maybe the reporting has some YEAST in it :)

Actually, it seems that the problem was reporting delay. I don't know why, but wix delivers traffic data with some odd delay.