Don't mean the max number of flights between two cities, I mean the Route Capacity that appears on the Office Tab. Number of total routes before you have to pay overtime comp.
Pretty sure it's a bug, if not the calculation is something I don't understand...
Had been allowed a max of 55 routes from my JFK hub. Max route capacity has just (or recently) dropped to max 45 routes, costing me some money in overtime comp. I haven't upgraded my base, so don't believe there's a gameplay reason for that to have happened.