I think the differences can be explained by the underlying assumption of a infinite canyon that @chris refers to.
Check this post Urban Heat Island in an “all-green” scenario for a similar attempt by another user at modeling a rural scenario. Relative to your scenario, he had some additional complexity with trees, which don’t apply to you, but you can see my explanation of some of the macro-parameters that assume urban conditions. You can modify them to get your UHI to a more reasonable level, but fundamentally, the UWG isn’t meant to model this type of pure rural condition with no built canyon.
S