hi carla
well, you don't have to!
if know how to do it, you can have mismatches between the page size in scribus and in the printer.
but you get into troubles, it's worth to first to get them right.
in your case, the problem is that -- obviously -- you did not get a matching double face printing, since the width of the page did not match. in this specific case, a difference in the height would not have done any harm...
ciao
a.l.e