[NTG-context] \setupbodyfont[10dd]

Hello, This question doesn't come from me, but as I wasn't able to answer, I'm posting it here: Consider the following text: \setupbodyfont[20pt] \starttext \input tufte \switchtobodyfont[10pt]\setupinterlinespace\page \input tufte \switchtobodyfont[10dd]\setupinterlinespace\page \input tufte \switchtobodyfont[10.6574803pt]\setupinterlinespace\page % = 10dd \input tufte \stoptext How are the interline distances determined? The last two pages give different results. The line breaking could be influenced by minimal rounding mistakes, but I don't understand why the distances between lines differ substantially. Thank you, Mojca