RPM is the number YouTube shows you
RPM is revenue per 1,000 views after YouTube has taken its share of the ad money - roughly 45%. It is the number on your YouTube Studio dashboard, and it is what you keep, not the gross ad revenue a brand sees. Because it is already net of the platform cut, multiplying your views by RPM directly gives the money you keep.
The calculator splits your total views by the share you mark as Shorts, prices each half at its own rate (long-form at the RPM you enter, Shorts at a lower one), and adds the two into the monthly figure. That same monthly number times twelve is the yearly total the page shows, so the long view comes from the same arithmetic rather than a separate shortcut.