Well, it’s based on a survey. (And surveys are cursed instruments [1].)
> Generally, owners' equivalent rent is obtained through surveys asking homeowners the following question: "If someone were to rent your home today, how much do you think it would rent for monthly, unfurnished and without utilities?" [2]
I would guess that some owners’ ideas of what their house would rent for might lag the market?
[1] https://carcinisation.com/2020/12/11/survey-chicken/
[2] https://www.investopedia.com/terms/o/owners-equivalent-rent....