So, SS13 is the most populated game on Byond.

Atleast thats what the front page says.

I'm pretty sure im no the first to notice, but the numbers just don't add up.

The page was saying 750+, while I only counted about 300 or so

So is this some bug that has been going on? Or is there something Im not adding up

Byond's Gang$ta
The hub does a degree of caching to the numbers so they're not always 100% accurate, but just to be sure I counted manually and SS13's numbers are a lot closer than your count seems to have gotten to.
I believe the full population includes servers that are off-Hub. Some servers go off-Hub while remaining active, and some like Lifeweb are permanently off the hub of Byond but have large player counts.