Z depth issue

I"m having, I think, a similar issue with Z depth.
I hope I’m not hijacking this post. but adding to it as it seems similar.

I"m prepping foam for cutting a wing part. I faced one side and flipped it to face the other. My final thickness should have been 1", but am around 3/4". not sure what happened.

I did zero out the Z axis after i flipped the part.

any ideas where to start trouble shooting?

Edit - thinking of it more, I think my issue is more about creating a precision stock part from an un-precise starting chunk of foam. Maybe I will start a separate thread on this.