What Critias said.
To reiterate (and hopefully clarify), in PACKS, you buy the attributes first, then you bring in the metatype.
So if you pick an attribute pack that increases AGI, CHA, INT, and LOG above 1 (almost all of them), you pay BP for those attributes.
Then, you choose Troll and those attributes get taken away, so the -10 BP bonus, as opposed to paying 40 BP to be a troll, refunds the BP you already spent increasing AGI, CHA, INT, and LOG.