In one of its groups, one client excluded some records with clause "equal to".
So basically the strange thing is that if you create a group with City = 7 or City = 3 and you add a condition "exclude" city = 7 then you won't find back the exact number of City = 3. It's not a really intuitive result! But there's a logical reason for it.
From the point of view of SQL DB, when a field is "NULL" as there can be in any DB, it is counted as "I don't know, it can be anything" not as "empty".
So when you "eclude City=7", the system also counts the "NULL" values because in SQL it means "it coulbe be, I don't know".
So the trick is not to use "Eclude City=7" but rather "Include City different than 7" and all will be fine.