I was thinking it's because I purchased the optional Hospitilers and mercenary for the garrison. Doing that cost about 7K and also 3K a month... the small print I didn't read either. If everyone is getting the 3K a town bill then that's good to know it wasn't buying the improved garrison.
It really urks me that NPC's get their initial garrison staffing immediately and it seems to take forever for the player's garrison to show up so I thought it was a nice compromise to be able to get some troops in there quickly. In hind sight though it's cost prohibitive. I don't give my lords Towns and I've got 15 of them now at 3K a piece and it's slowly sending me to ruin.