Download speed and actual speed

[#1]

When I download through firefox from a good server I get speeds of 500kb/sec but when I do a speed test at mybroadbandspeed.co.uk the resulting download speeds are 3000kbps there or there about and I was just thinking shouldn't I get these speeds when I download

[#2]

Have a look here about the rate limits from certain download sites http://www.plus.net/support/broadband/quality_broadband/speed.shtml