[NTG-context] Question about text offset

Dear list, I was wondering, why is the text in the main text block offset by .25\exheight from above (and below)?  Also, can you point me to the source line where this offset is added in shipout because it is not there in \setuplayout.  If you don't know what I mean, see MWE below. Cheers, Henri --- \definelayer   [pagebg]   [width=\textwidth,    height=\textheight] \setupbackgrounds   [text][background=pagebg] \showframe \starttext \setlayer   [pagebg]   [voffset=.25\exheight] % why?   {\vbox{\strut\input lorem\par}} \input lorem\par \stoptext