Perhaps your gun had an adjustable choke (thus unmarked) and it's had the barrel shortened.
For some reason remington 870, 1100, and 11-87's quoted barrel lengths with vent ribs are rounded off to the closest inch. Every factory barrel I've ever measured was like that. A factory 26" barrel will measure 25.5", 28" will measure 27.5". One of the 23" supermag turkey barrel's measured 23.5". It is possble someone shortened a longer barrel and cut it at the post, but it is also quite possible the barrel has not been cut.
Just a guess here but I'm thinking they make all of them to 30"-32" to start then cut them down to whatever final length they want before threading for choke tubes. In order to use the same vent rib on all models they have cut the barrel so that the closest vent rib post matches the end of the barrel.