If you set your Google Calendar's event to be repeated monthly at 31st day of each month, then what would be more reasonable in your opinion:
- repeat it monthly on every last day of a month -- 31st day of each month, that has 31 days and on 30th (28th or 29th in February) for other months,
- repeat it "monthly" on every 31st day of each month, that has 31 days and do not repeat it on every other month, that doesn't have that much days.
I don't know any person, who would pick second option. How can you say "repeated monthly", if it occurs only five times per year?
Unfortunately, due to Google Calendar engineers stupidity, second option is, what actually happens in current version of Google Calendar.