Because certain studios, most notoriously Toei with 
Dragon Ball and Sailor Moon material, had/have a 
habit of just throwing any ol' background from a 
series with a cel, regardless of whether or not the 
cel and bg match.  Add to this the facts that shops 
in Japan will just sell you a sack o' backgrounds 
for a few bucks and that there are always more cels 
around than backgrounds, and you've got a bit of a 
problem.  Most dealers don't have a way of knowing 
if the bg necessarily matches the cel, since the 
only real way to tell if the two match is to be 
familiar with the sequence from which the cel is 
taken, and most dealers can't afford that kinda 
level of familiarity with any given series.  It's 
not that rare to find original matching bgs with 
cels depending on what show the cel is from and the 
studio that produced it--I'd say close to half of 
my 60  Battle Athletes cels have their matching 
BGs.  Reputable dealers will often note if the BG 
might not be matching (or is a laser copy).  Some 
cel owners will just make their own bgs or use 
stray ones just for display purposes; they'll 
usually note this in their galleries. Presence of a 
matching bg usually adds some value to cels, but 
not always.

So how to verify you got the original matching bg?  
Well, some studios (AIC) will put a stamp in the 
corner.  That'd verify it's authentic but not 
necessarily that it's the matching one.  To check 
if it really matches, well...you just have to know 
your series inside and out.