I'm not sure what you'd expect the function to return when the number is not a divisor of the number of units in the implied containing period.

For example, if we have an interval of months, we are typically looking at how they repeat within a year, and it makes sense to have n as 1,2,3,4 or 6.  So, for example, 3 months = 1 quarter and we get 4 per year.  But if we asked for a period of 5 months then the generated dates will not be aligned to a year and the results will not have an obvious meaning.  Likewise we might be looking at minutes within an hour, so 1,2,3,4,5,6,10,12,15,20,30 make sense; for example to report an activity for every 5 minutes within the hour makes sense, but asking for the value in groups of 13 minutes would not as it wouldn't repeat each hour.

In your example, 3 quarters (9 months) is not a divisor of a year, aka 4 quarters aka 12 months, so the result has no intuitive meaning.

I should have added this as a conceptual limitation of the function.