Yeah, so?
MS does at least a build per day during development of new OSes.
They would already have been on build 2506 or 2507 when RC1 hit the street.
Add all the builds done in the weeks since it went out, and the internal builds
should be around 2530 by now.
From what I've heard, 2520 is pre-RC2, it was sent out to testers to make sure
MS is on track to release RC2 by the end of the month.