The discrepancy between the number of members displayed in Outlook and OWA could be due to different ways of counting the members in the group. It is possible that Outlook is counting only the direct members of the group, while OWA is counting both the direct members and the indirect members (such as members of nested groups).
Another possible explanation is that there are guest users or members from external domains who are part of the group. These users may not be displayed in Outlook, but are included in the count in OWA.
It is also possible that there is a bug in either Outlook or OWA that is causing this discrepancy. I recommend checking with Microsoft support for more information and to see if there is a solution or workaround available.