I got 15 moderator points today for the second time this month. Last time it happened was two weeks ago, May 5'th. Apparently I'm not the only one that has gotten 15.
I think what is happening is this:
If you post too often, it won't give you mods points. However, if you stop reading slashdot (like over the weekend) the mod points it would have awarded you are saved up and you get them the next time you log in after the break. I'm not the only one that has had this happen, I saw a journal entry mentioning the same phenomenon but my Google-fu is lacking today and I can't find it.