Autosomal DNA results like those that are provided by FamilyTreeDNA FamilyFinder, 23andMe, and Ancestry.com are not phased.    The individual base pairs (AC, GT, etc) are not ordered but listed usually in alphabetical order.  

So just because you may have:

You Your Match
AC
GT
GA		 AA
GA
GC

All the major companies would show the above as a match since there is one matching nucleotide (letter) in each line.

However, that may be just a coincidence (IBS: Identical by State).   One way to narrow down the odds is to phase your DNA.

 

You Your Match
Your Mom    Your Dad  
A
T
G		 C
G
A		 AA
GA
GC

After phasing your DNA you can now see that your match isn't really a match after all.   You can phase DNA only partially.   Doing so requires at least one other relative.   The more relatives and the closer the relation the more DNA that can be phased.