About the difference in simulated and measured antenna radiation patterns

I designed two slotted waveguide antenna to realize pencil and difference beams.
The simulated and measured radiation patterns are matched well within +/-90 degee,
but outside the range, they have a big difference. Please see the attachment.
Is it reasonable? Can anyone help me to explain this?
really appreciate!

Yes. It means you can believe the measured data, and that the predicted data is wrong.

What I can see in your diagrams is a poor side and rear transmission while the main lobe looks good.
I would suggest to put a metal-sheet screen on the back of your waveguide with slots and test the setup again. The screen will suppress the problematic side and rear lobes.
Waveguide slot antennas are rarely used bare; often they are used as primary radiators in parabolic reflectors which then suppress the side and rear radiation.