stat.damageTaken ALWAYS increases in lava

I was playing around with the stat.damageTaken objective, and I started thinking how it worked (with one hit from a spider it increased with 20). I thought "maybe it checks for how long I'm in a spot in which I can take damage", so I checked it with lava and fire resistance.

How to recreate:

I used /setblock to create a floating lava source block. I gave myself fire resistance, and walked into the lava. I had the stat.damageTaken objective on my sidebar.