Calendar year colors

I can't seem to find any rhyme or reason why some event boxes are yellow and others are orange. I went back and forth between "Month" and "Year" views, trying to figure it out, and based on the events, I just can't seem to figure out why some events would be orange whilst most are yellow. It's not different calendars, it's not all-day vs. timed events, it's not reacurring events... all those have been elimated as possible reasons. I just can't figure out why some are orange, it's really puzzling me and I was wondering if someone here knows why?