How can a read only MYISAM table become corrupt?

I have to do a myisamchk -r approx. once a week as the table gets marked as crashed.

I can't see how this is happening though as no changes are being made to it, it's read only.

It's running on mysqld-nt version 5.0.51b, the same table running on a mac and a linux box have not done this, so far.