So I found a helm with 177 health. When I change my current helm with 0 health to this one I go from 7581 to 7571, so actually 10 health less.
I checked every other stat on them and they have no other differences in health.
But when I change my boots to a new pair and back again my health goes up to 8174 so now the health buff from the helmet is calculated correctly.

I then found a chest piece with 425 health instead of the one I had before with 189 health.
I use that one and get 8590 health.

But when I swap back to the 189 health chest piece it goes down to 7537 instead of 8174.

I then do the boots trick again and swap them and are back to 8174 health as it should be.