something's puzzling me.

I have a modest but very dependable 8meg Broadband connection. Had it for years and have no trouble watching HD TV, playing online and attaching our phones to it simultaneously.
If I test this using any of the sites offering to do so I get anything twixt 7 and 9 MB/S.

If I download a file from Steam this usually maxes out at 1.1 MB/S.
During the download everything else slows and a check on the same test sites tells me my speed is 512KB/s-ish.

Where's the 6 or 7meg left gone? Am I being thick here?