Among the settings on the criteria sets are two that allow you to determine if you want to match to records with blank values and if all file fields must have a value.
While older versions of Importacular may not have these checked, most use case scenarios will function best if they are checked. When they are not checked, Importacular will match your data based on the populated fields only. For example, if your criteria set matches based on first name and email address, but does not check both of these boxes, in a case where there is no email address to match to, Importacular will seek to match to just the first name.