MIDI messages and jitter graphics

I've started experimenting with Jitter, using the tutorials as a foundation and have gotten to a point where I can mix multiple video sources as well as using a mask so I can produce circular output.
I set up the mask so that I can change the radius so as to show more of the content as desired and I'm using a slider to define the radius. Wors just fine.

Next I wanted to use note numbers from a keyboard to control that radius so I fed an appropriately scaled note number into the slider. When I play notes, the slider changes as expected but the video results didn't change.

Eventually I discovered that if I put a [deferlow] between the incoming note and the slider, then the video works properly.

I don't quite understand what is going on. I know there are high and low priority threads and I presume that [deferlow] causes the MIDI event to be posted to the GUI thread. But I'm wondering what happens to the MIDI events that are sent directly and being ignored. I didn't see any error or warning messages in the Max window. Are those events being cleaned up properly?