Warpping direction

[XprinterX]

Good morning!

I have been working lately with additive manufacturing and I have faced a question that I couldn't answer reading on internet.

I know that warpping is due to a difference of temperature between the bed and the new layer or between new layers. But why does plastic warp edges upwards?

I mean:



and



From what I know, cold parts (bed) should contract and hot (new layer) expand so if we look at the piece it should be contracted in the bottom part, none in the neutral axis and expand on the upper part as it is hotter.
I thought it could be related to he infill but, as it happens in the photos I posted, the parts are flat so no much infill.

What am I missing to understand this?