strange ?

I installed ( to learn ) my own setup class installer and device.

Independently ( for reason of entertainment ! ) I began has to
install Winwows Media Player 11.

And surprise ! ( for me ) My class installer ( myclassinstaller.dll)
receives twice DIF_ messages (*).

(*) I placed in Entry Point of myclassinstaller.dll a MessageBox
informing me messages which it receives.

What is the explanation?

Thank.