If there is other traffic on your connection then it can give misleading results.
To be certain of accurate results, you need to make sure that no other devices are using the connection - in other words they should be switched off, and on the device running the speed check, other programs must not be using the internet and no automatic updates must be running.
It's sometimes also wise to be sure no other programs are running that may be using significant amounts of CPU or Disk usage.
And sometimes some AV and Anti-spam programs can interfere with such tests, so a repeat check should be carried out with them disabled if the original checks are below expectations. And of course the connection should be wired, not wireless to rule out any wireless issues. HTH.
