If I use 72 minutes for Havdalah, why are Havdalah times ~90 minutes later than the previous day’s candle-lighting time?

Shabbat begins 18 minutes before sundown on Friday night.

Havdalah is 72 minutes after sundown on Saturday night.

So in a given week, Havdalah time is typically 90 minutes after the previous day’s candle-lighting time (18 + 72 = 90). Sometimes there’s an extra minute or two difference, and that’s due to sunset actually differing by a minute or two between Friday and Saturday nights.