Different MIDI Velocity settings for different instruments?

[Petr Pařízek]

Hello,

I made a LilyPond score for piano and 2 flutes.
When I examined the MIDI file, I discovered that all the Note On 
commands in the piano part had the Velocity parameter set to 90 while in 
both flute parts it was set constant at 62.
May I know why different Velocity values are being used when I did not 
specify any requirement of that kind in my source ".ly" file?

Thank you.

Petr