Answer:

When Alloy says "no counterexample found", that means the
assertion is definitely true within the chosen scope.

Alloy Analyzer doesn't know whether the assertion is still valid
with a different scope, and that's why Alloy Analyzer only
says the assertion "may" be valid.

Basically, it means we did not find any example where the assertion fails.
The assertion may still be false in general, but Alloy Analyzer
could not find any example where your assertion is false
within the scope you specified.