Having fast internet speed is completely different to downloading a file from the internet.
Your internet speed is only the speed between your house and your internet provider (ISP).
But when you download a file, the route goes much further, see below:
House > your ISP > ISP of opposite side > Server
The first question is, how fast is your internet speed really? By doing a speedtest as you did, you follow the same route, where the ISP of the opposite side and their server are where the speedtest takes place. These are usually high end servers and their network are also high capacity networks designed to measure your network speed and thus allowing you a really fast speed.
There is a rule of thumb though, the further the server is away, the longer the route becomes. Data is chunked into several sections, and each section is sent over the internet in a separate package. It is possible for one packet to become lost or delayed which means that your transfer will be slowed down.
This could be the case with steam, as the server might be far away from you. Steam has many servers and you can connect to a different one, increasing the chance of a fast download.
It is also possible that the steam server itself just doesn't have the capacity to handle your download, or they slowed down to give other downloads more priority (most likely the case)
It is even possible with steam, that on one download location, the server gives priority to a certain download, while on another download location, they don't. My suggestion therefor would be to first try and change to a different download location. I've had weird results choosing a different download location, one that was even further away, got me a huge download speed increase.
