MIDI Tempo

Hi all.

I have a short score in 6/8 time. I attached a tempo of 60 crotchets (quater notes) per min and exported as MIDI.
I then changed the tempo to 120 crotchets per min and exported as MIDI again.

When I print out the MIDI files generated by MuseScore, there appears to be an issue with the tempo. The two files have different tempos, but not different enough. I was expecting one tempo to be twice the other, but it only seems to be about 1.28 X.

In Java I can print out the length of the MIDI file in microseconds - this agrees with the tempo messages in the file, but it is still not twice.
Also when I play the files in Windows Media Player, they play VERY slowly!

Am I missing something, or is this a bug?
Nightly build: 5659. Windows Vista 32-bit.

Cheers
Don.