Lower throughput only on speedtester

[#1]

Hi All,
I am trying to explain this phenomenon:
When I run a speedtest early in the morning or late in the evening when there is not much contention or throttling, it struggles to get above say 5.7Mbps.

Immediately after I would download a big file (say more than 100MB) and monitor my download speed:

Total rates:  6105.9 kbits/sec
Incoming rates:  5995.9 kbits/sec

This has happened more than a few times, enough to raise my curiosity.
My question then is, why online speedtests reliably show download speeds of around the 5.7Mbps range, but when downloading a file I register speeds of 6.1 - 6.2Mbps?
Have you noticed something similar with your line speeds?
PS.  This is my modem details:

ADSL Link	     Downstream	 Upstream
Connection Speed	  6976	      448 kbps
Line Attenuation     39 db	   12.5 db
Noise Margin          5 db	   23 db

[#2]

The speedtest measures the speed between you and the server at a moment in time. Your file download speed is the same the speed between you and the server.
Its like driving to London from your house you could go on the motorway and have a clear run with no traffic or go on the A roads and get stuck in traffic. Its the same on the internet the speedtest is only measured on a short burst while a download is a longer test.
Some download sites are better as they have a good connection and infrastructure to match. Just like going on the motorway you can have good times and bad times any test is only a guide

[#3]

Thanks samuria,
What surprised me was that the difference has been pretty reliable.  Late at night or early in the morning there isn't much contention (there's never high contention at my exchange anyway) and all I could think of was that packets are being dropped by the ISP's traffic management switches.  Speed tests at busy times can be more than 0.5-1 Mbps lower.
On a similar note, how come I always end up on gateways with high packet attrition; like now for example, I am on ptn-ag1.  Somehow, I rarely manage to stay for any length of time on pcl-ag01.