The narrower environment does also use font dependent values for the left and right margins.
A good method to set a global value for all environments is to set the dimension with the
\definemeasure command and access it with the \measure command.
% engine=pdftex
\definemeasure[myindent][12pt]
\setupindenting[yes,\measure{myindent}]
\setupnarrower
[ left=\measure{myindent},
right=\measure{myindent},
middle=\measure{myindent}]
\setupitemize[each][margin=\measure{myindent}]%,width=\measure{myindent}]
\starttext
\input knuth
\startitemize
\item \input ward
\stopitemize
\startnarrower
\input zapf
\stopnarrower
\stoptext