It doesn't matter ... you can actually contract MORE than 600B per month however, once you spend any money and lower your spending limit for the month if the game sees that it will cut your contracts. This is easy to do with anything being automatically ordered on immediate. Unfortunetly even if you catch the order and cancel it, the money is not spent but the buying space is. (another W3C fix done to frustrate some players)
Most likely cause for that is in the case of weapons contracts, consumption purchases or manually entered buy orders.
i.e. you have 600B per month spending and you purchase 300B that only leaves you 300B if the total of the contracts is above 300B contract amounts are pared down slowly.
If you want to rely on contracts you have to stay away from manual purchasing, and really fine tune your buying strategies.
Use to be players would offer MORE than a company was able to produce to compensate for that some ... W3C stopped that by limiting how much you can offer.
