When LE agencies buy firearms, they do so in batches. Typically they buy more than they need, in a bare bones sense. The extra are for loaners during repair, for new hires, etc. The ones that remain in the armory unissued can come up as surplus as the agency changes to a newer era of firearms. Could be this is what your 15 is.
Some LE divisions of LE seldom carry firearms. For example, the internal affairs/professional standards division. This is especially true if they are inside a jail/prison. Those handguns get shot only during quals and go right back into storage until the next qual.
A really nice 15; enjoy it.