If you're using all services, then any partial outage is essentially a full outage.
Of course, you can massage the numbers to make it look nicer in the way you described but the conservative approach is better for the customers.
If you insist, one could create this metric for selected services only to "better reflect users".

That being said, even when looking at the split uptimes, you'd have to do a very skewed weighting to achieve a number with more than one 9.