Serial No. xx ?????

Hi guys, as a newbie, I've noticed one puzzling thing about some of our correspondence here and some of the Lugers described in ads. A lot of times it appears to me that someone only reveals the first 2 digits of a four-digit serial number. "... 1917 DWM, serial no. 45xx..." I've seen this a lot, and wonder what the reason for it is. Is there a concern about revealing the PRECISE serial number for some reason? Thanks for any enlightenment on this!