|All network related devices - network adapters, high speed modems, routers, etc. - are rated in bits per second. The same applies to your ISP's max rated speed. |
There are 8 bits per byte, but when you take into account error correction and other networking overhead, you end up with a rate in bytes per second that is about one tenth of the rate in bits per second, not one eighth of the rate in bits per second.
You'll get different max ratings at a particular time depending on what is between you and the server where the particular speed test is run. Choose a site where you get a higher result, and stick to using that site and the same test for testing your speed for comparison purposes when you want to check your speed at various times.
If you run the same download speed test twice or more in a row, the second or more test usually yields a faster result because part of the data downloaded for the test is already on your computer - the first result is the most accurate one.
E.g. I use a speed test at www.toast.net, and I get the best result when I choose a server that is in Canada (I'm in Canada).