Magic Mask render not matching selection

Can someone explain why the actual render of this selection looks like this? (My magic mask overlay is attached below as well)

When my magic mask is perfect:


You can see the settings of the matte at the bottom. I haven't engaged any kind of softening on the matte. Shrinking or enlarging the matte just makes the blurry matte smaller or larger. Nothing can be done to tighten or make the edge match the actual tracked matte that it shows in the red overlay.

I'm not a Magic Mask guy, so maybe someone will come along with a better answer than this.... but in my lurking on Magic Mask threads what I've heard is that Magic Mask is not a precise tool and it's not something you should try to use as a substitute for a green-screen-like effect. It's more designed as a colorist tool for local modification of color balance or tone curves, where imperfections in the mask aren't really noticeable.

Someone correct me if I'm wrong here.

Hey
I've intensively used the MM and this doesn't belong to the bunch of issues I've encountered so far (since 90% is related to the cache anyway).
It indeed looks like there's a blur parameter on, which is not obviously (BTW, sure you didn't put any blur on a node around ?). More, if I drag strokes on fingers, the MM retains it and I've had pretty good results with the MM despite some opinions of preciseness I've read here and there, but here we can see the mask is done like a polygon around the whole hand, not taking the spaces between fingers into account...
So yes, this mask is not satisfying....
Personally I use it in Fusion so I can fix edges anyway but here, though I know I could try an erode/dilate node and so tweaks in a MatteControl node, I'd try to make it again, with strokes at the beginning, others at say 60% of the clip, both with strokes on fingers...

Writing about Fusion, when I see the bed sheet which is really blue and thereby remote from the arm hue and gamma, depending on the length of the clip, I'm pretty sure I could get the arm selection with a key as well