Does your technique "really" improve or...

I would go with technique. If your face toughened up from the rigors of shaving, then everyone who used a cartridge would look like leather

 

I cast my vote for improved technique. When I first started there was blood everywhere. Too much pressure. Now I let the razor head do the work. Have come down from a 3 pass shave to a 2 and a touch up.

 

Yup. As others have noted, you have just improved whether you know it or not. Your skin on your face doesn't toughen up in the same way hands toughen up with callouses.

 

I vote voodoo magic... man

 

I do think most of the confident footing comes from technique. But I also do think skin health comes into play, and using a cartridge razor and associated gear does not help your skin.
This last year I started using PSO and my shaves are a clear step above as a result. My skin is better hydrated, tight and elastic. It resists cuts better and recovers from rough treatment a lot faster, giving me a wider range. I can now do three passes with even my most aggressive gear without risk.