These two logs below show that a nerve to the knee doesn't always result in movement loss. So to start you have a 1/4 chance to hit the knee and then it doesn't guarantee movement loss. I'd say it's just a tactic that worked out well for the Crraya in this case. Plus, it definitely isn't the only thing that killed you. I mean she hit your knee 3 times out of what 14 or 15 nerve strikes?
Also, in the example I showed below the -mv loss was 155 points. I'm sure there is a range but if that's 20% of your movement that means you have about 750 total movement points. That's not all that high. Maybe try exploring some?
wilderness <1013hp 711m 1188mv 26000tnl (-24.40%) 8 PM> poisondarts sleep ser
Serra steps out of the shadows.
Serra kicks at the inside of your knee.
Serra's pressure point strike wounds you.
You yell 'Help! Serra is trying to strike me!'
A massive giant is convulsing on the ground.
wilderness <994hp 711m 1188mv 26000tnl (-24.40%) 8 PM>
816/891HP 519/673M 945/960MV 24350TNL wilderness raining 8 PM -
Ishuntal kicks at the inside of your knee.
Ishuntal's pressure point strike decimates you!
Ishuntal has a few scratches.
790/891HP 519/673M 945/960MV 24350TNL wilderness raining 8 PM -