Im on a 10mb deal from Virgin Media (rated at 9.5mb by broadbandspeedtest) which should roughly equate to download speeds of 1.2mb/s.
When I download games off of Steam, I get the 1.2mb/s for abit the first half hour to an hour, then it will suddenly drop to around 250kb/s and hover around that area for the remainder of the download.
It can be just about anything. That 10Mb may be for burst mode which will use full throttle for the first few minutes then slow down. The steam servers may be congested. Any router or hardware your data has to travel through on it's way from seam servers to your house may be congested or just not working right.
Basically, your fastest speed is determined by the slowest link in the chain. The internet doesn't create direct connections between two computers. there are may routers and networks it needs to go through to reach it's destination.