Ask HN: How common is this Adsense Weirdness?

I track my negligible Adsense earnings every now and then. In fact, every month or so. Last month, I was glad to learn that the famous $100 limit was near. I had $96 or thereabouts. Hallelujah!

Considering that I get around 500 visitors daily on my site, I log in today to check whether the $100 limit was crossed, and I find that somehow, Google has recalculated and now the Adsense figure stands at 87-odd dollars.

Now I am pretty sure that the Porsche will have to wait.

What also pisses me off is the sheer Opaqueness of Google. Does this sort of Adsense weirdness happen to others?

Thank you.