Some people feel the need to hide their full serial number when posting. You will notice that most gun magazines do the same. For instance:
"I have a Colt 1911 in .38 Super, Serial Number 010XXX. It is in near mint condition, and was a given to me by my aunt when my favorite uncle, the original owner, passed away. Can you tell me if it has any collectors value,"
The author of the letter will include the entire serial number, when he writes to the magazine, but what gets printed in the magazine has the last 2-3 numbers "X"ed out to prevent any number of mis-deeds. Basically, it is an unlikely crime that someone will report your gun by serial number as having been stolen from him. and easily disproved, but the magazines and some posters here would rather err on the side of caution.
Anyways, thats why you see the X's.