Hi @Janet1748
Airbnb no longer has a setting for custom trip lengths (it's been scrapped) and 'gap nights' are a feature on some other sites but not Airbnb.
The current Airbnb interface overrides your global minimum in one of 3 ways:
1) You may have set a minimum stay for a specific night of the week. Click through on your global minimum night setting to check whether all days of the week are set to 2 nights.
2) You may have clicked a specific night on the calendar. A screen pops up where there are custom settings, one option is to change the minimum for that night only.
3) A rule could override the minimum setting, but I don't think you've made a professional hosting tools rule set (you would've known if you did this).
Any of the above could also have been done by a co-host, if someone else has access to the listing.