There are a couple of ways this can happen. They are based on the principle that an assertion "passes" if IFV cannot find a sequence in which the assertion reaches an error state.
So, if you have an assertion that looks like "trigger_sequence |-> suffix_sequence", then the assertion will "pass" if the trigger_sequence can never occur. Since the trigger can't occur, the suffix doesn't need to be checked. This is by far the most common way to get the results you have described.
You can check to see if the trigger is possible, given your constraints, by issuing the TCL command "define witness_check trigger".