Why is the difference in displacement so big?

Hello everyone,

i am analysing different methods for bolt modeling in HyperWorks and I found a big difference in displacement for two actually very similar models.

The models contain two plates with a bolted connection with pretension and an external force. The only difference between them is that for one model the plates consist of 2D Elements and for the other it is 3D Elements, therefore I expected fairly similar results.

I found that in case of 2D Elements for the plates the pretension force is what causes the drop in displacement, but I dont understand why, it feels wrong.

Can someone explain this? The model files are attached, Thanks.