@Will_Peters is correct that Alternates are ‘approved’ by Adaptive Training to fall into what’s suggested for that day. (Thanks, Will!).
I just want to address specifically your question about Alternates showing differences in ‘difficulty’.
Alternate workouts may have different TSS and IF than the original workout, which may result in an Alternate being slightly easier or harder than the original.
You’ll always be presented with an Alternate in the same training zone and workout profile as your original workout, so you can be confident that choosing an Alternate will still effectively move you toward your goals!
More info on Workout Alternates here, but don’t hesitate to reach out if you have further questions!