Speed tests can vary quite a bit depending on how they are conducted. Some ISP will recognise that you are going to a testing site, (they do know everywhere you visit after all, unless you use a VPN), and they will 'prioritise' your data stream. Not all can be accused of this of course. The actual data stream used for measurement can be optimised as well. For example, if you use the NETFLIX speed checker, the data will be representative of video streaming, which is quite valid of course, it is what they do. If you use speedtest.net, their data seems to be optimised for the best connection to the testing server from your IP. On the other hand, if your speed feels adequate, there is no point in measuring it to the last decimal point.
Also, your perceived speed is determined to a large degree by the site you are connected to, usually OK, but there are some snails out there.
Andrew.