If the speedtest file is too small, then it can skew the results, and make things appear to come down quicker than what they actually do.
Theres also something else that sometimes happens with 'speedtest files', and thats when your browser starts to 'cache' a part of the file before download is acknowledged, and once youve said 'yes save file to here', it appears to come down very quick for a very short period, but would eventually level off.
When you connect to a decent speedtester its meant to perform a very quick test to get a rough indication of your speed .. and then send you the right file size.
If during the 'setup' test it detected a slower connection for some reason, it could end up sending the wrong size test file ie one thats too small to accurately test.
