I believe server sends a gziped version of the file, browser downloads the gzip'ed version and unpacks it.
I can assume that 100mb.bin file gziped is less than 100kb size hence why you get that download speed. 100kb downloads in less of a second, but since the calculations are done considering 100mb size it calculates a much higher download speed.
This does not happen with Internet Download Manager as it requests the file several times and with no encoding in place.
i think that we found the answer!