Please help me to understand how UPM is related to ascenders and descenders. For example, look at Times New Roman (TTF) metrics, UPM = 2048 (see image below).

I used to believe that UPM = Ascender - Descender, but now I find that I was wrong. But what metrics expression must be equal to UPM? What plus what (or minus what) must give exactly 2048 in this example? I don’t see appropriate values at all. Help me to clarify, please.