The manual also mentions that it's time-based too.

From the manual: "Note: Oil life start value 100% equals 10,000 miles (16,000km) or 12 months. For example, setting oil life start value to 60% sets the oil life start value to 6,000 miles (9,600km) and 219 days."

Looks like there must be some combination of the two - so maybe when it sat for a while the % was time-driven and then after she got it and drove it around it started to become milage-driven. Or maybe it's driven by whichever function is governing at the time - if you drive a lot of miles and would drive more than 10k in a year, it's milage driven; if you don't drive a lot of miles it would be time driven. "10,000 miles or 12 months, whichever comes first."