The 6-month-active count usually means “This user has been active at least once in the last 6 months” not “This user has been active for over 6 months”.
So if you sign up today and spend today on Lemmy, then you have been active at least once in the last 6 months.
Fledditors are all new users that by could not be reflected as active for 6+ months due to their accounts being ~2-3 weeks old.
The 6-month-active count usually means “This user has been active at least once in the last 6 months” not “This user has been active for over 6 months”.
So if you sign up today and spend today on Lemmy, then you have been active at least once in the last 6 months.