tribute questions

In yesterdays game, i was lucky to find 3 minor races early on in the game. I changed my stategy and traded everything with them for monthly tribute. Near the end it was nearly 6000bc a month, and all the minor races had either defected to me or had been conquered by the a.i's. So why was the tribute still showing up? Also if you trade for a monthly amount (like 100 months) and the trade goes green, you can then change it to 990 months (haven't tried farther than that yet) and the trade will still go thru. That doesn't seem right.