Does Steam cap downloading speeds?
yes. though i wouldnt know the details.
like if certain games/files are more popular. and on a particular server for steam people to download and install from. they might limit all the users less total speeds each. if people are capping off at that speed, they cant lag each other's downloads by using up all the bandwidth.
while less popular downloads on less traffic locations, they may allow you to download way faster.
if a new game is released and is more popular then steam tech's predicted, they may not have allotted enough bandwidth for every user to reach the cap at all.
meaning all the users lag each other and everyone suffers for a day or 2 till the hype dies down.
your internet speed means nothing in these cases. being able to reach "steam's" cap is the best you can hope for.