well, when series were still done with cels the
backgrounds used on the series were also hand
painted, and if i'm not mistaken many of them are
still hand painted. Original backgrounds are rarer
than cels because there's ussually only one for
the entire sequence of cels, and only one of the
cels will be paired with it (typically the first
or last)

Anyway, you can be sure that non matching
backgrounds are not mass produced to make cels
more attractive, people just place them together
because of the effect (result is sometimes good,
sometimes not as good, it all depends on the taste
of the person.). Some cels "need" backgrounds
while others are already good without any.