What is the reason that SendToModule and some other stuff pauses when menus are open?

I have a background program running that sends updated info to one of my FvwmScripts to display. One day I noticed that it didn’t get updated while I was in the menu. What exactly is the reason for this? I am interested in knowing the details behind it, even if there isn’t an easy solution

It’s because various things get grabbed.

This tells me nothing, as it is too vague
Why are FvwmScripts able to update themselves while not being able to receive input from outside? Does fvwm use a common synchronous handler for a lot of things?
I hope I’m not too much of a bother