36" window in a 51" wall--why not?

[hoodatiz]

Help--must be missing something. Trying to build a bow wall out of 5 segmented walls turning at 30 degrees. Chief will not let me enter a window width of 36-3/16" wide; certainly it's not a question of trimmers--is it? Looking at the help index, I can't find a quick answer. Probably a stupid question, but what am I missing in order to place this std width in a much wider wall?

Thanks in advance--Don