In reply to Andre.

Andre -- This is not an official answer, but I spot-checked a bunch of themes in the 2,000-4,000 range, and found no 500s.

Browsing "popular" themes...
D5 CORPORATE LITE is 3,000+
The very next theme down is Voyage with 2,000+

Enigma has 30,000+
Minamaze has 20,000+ (next theme down)

So it appears that themes 10,000 and under are rounded to 1,000s, while themes over 10,000 active installs are counted in increments of 10,000.

I would guess the same is true for the hundreds of thousands, though I didn't check.