Looks like you have a bolt in the center that is keeping the assembly together. I suspect the higher stiffness is because of the contacts you’ve given. Here are the contact definitons you should give:
If you have any pretension defined, make sure the axis of pretension doesn’t coincide with defined bonded contact region.
To get more familiarised with contacts and modelling bolted connections, these links will help you.
Definition Settings (ansys.com)
Preloaded Bolted Joint Analysis | Ansys Innovation Courses
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