[deater]

It's likely it's 3 bytes of BCD (binary coded decimal). It's a handy way of storing things like scores, especially on 6502 processors which have built-in BCD handling support.

[hcs]

No BCD support on the specific chip used in the NES, though.

[tom_]

Looks like it might be doing the bcd by hand. See, e.g., L93C7 (and following code) in the disassembly here: https://github.com/CelestialAmber/TetrisNESDisasm