Those erroneous speed results are generally the result of caching by 'web guard' antivirus products rather than the browser. They collect the received data into 'pools' which are examined then released to the system. It happens so fast you'd never know about it... until you try to run a speed test and the results are far faster than you'd expect!
Basically your AV simply collected the incoming data (from the test) and analysed it before releasing a couple of seconds' worth of downloaded information to you at once; thus making it appear to be ~100Mbps. Temporarily disabling your AV during a speed test will eliminate this issue. Just remember to turn it back on!
