The problem is two fold. 

1. Your character did not dive away from the grab but toward it so the likelihood of it completely avoiding the grab is unlikely because of how long it stuck around. 

2. As you astutely pointed out the A.S.P does not have a head and the vulnerable point your character attacked is front facing (see reference pic). Attempt to sensually strangle yourself and you'll have a better idea of the gap Slimy is trying to 'exploit', the gap that forms in the arm in that movement is more like the one forming in the Mech's arm. 

Anyway, I'll just post and let that do the explaining, as you're committed to diving through the Mech's arm.