Hello, I need advice of peoples that already rebuilded an engine of 190e and more specificly 16vlv if possible.

A couple of days ago I removed the head and saw that the piston gap on the top of the piston was about 0.5mm (I tought that wasnt normal). So I deceided to remove pistons to measure everything.
The cylinders have an average diameter of 95.52±0.03 mm.
The pisonts have a diamater of 94.65mm and 94.44mm (on each axe) on the top of them and a diameter of 95.45mm on the botom and middle of them.
So I touth that the pistons where worn since their diameter at their top where 1mm under the one of the spec.

So I deceided to buy some new ones. And it seems that the new diameter is:
94.44 and 94.65mm (on the both axe) on the top of the piston, and 95.44mm on the middle and botom of the piston...
So the measure are exactly the same....!!!!

So it seems that it is normal that the upper part of the piston is 1mm smaller in diameter then the rest of the spec (95.45mm).
My question is: How come???
A gap of 0.5mm-1mm isnt too much to allow some fuel to be cought in that gap and disable it to burn during the explosion?? That doesnt seem logical to me...
Can someone explaine me why?
I can even see a bit of blow by on the old piston, isnt that an evidance that the gap is too big??

The other thing is that I have some unburn fuel that is passing trew the engine, and I cant imagine where it can escape the burst of fire during explosion?? I noticed that my rings are loose on my old piston if I compare to the new one, could it be the source of the problem??

I really need advice of specialized and experienced member please...

Thanks you!