Dipole impedance depends on air box size with HFSS

I have made s simple dipole with lumped port at center and variable air box size.

The impedance changes quite a bit versus air box size.

I think HFSS recommends at least 1/4 lambda from antenna element.

Zreal changes from 82 ohm to 95 ohm when size increases from 1/4 to 1 lambda at 1GHz.

The minimum Zreal at 82 ohm is when size is exactly 1/4 lambda.

Theoretical Zreal is 73 ohm.

Could somebody please explain to me why Zreal changes so much when size increases?

I cannot get down to 73 ohm, any suggestion?

Thanks.