I've noticed that if a player (or players) spectate, then the server browser detects them as being "non-human" (every time I have joined one of these servers, the amount of players which were not detected "as human" by the server browser have always been equal to the amount spectating at that time, and the player count/capacity suggests the same).
Also, some servers are always so full, that there never needs to be bots in there, so for bots to have been in prior wouldn't have happened! Just an observation. Still, it's a useful feature for when you see 5 players, yet 0 when the option is turned on!