No. Function, quality, reliability.
Put simply, Police Magnums (and Wingmasters) are better finished and use better parts in some places than 870 Express shotguns. More hands-on labor (finishing), better processes [park (PM) or deep blue (WM) vs Express finish] and better/stronger/more expensive parts (metal vs plastic). Sure, an 870 Express can function, but it is not going to do so as smoothly out of the box Also, down the road, under hard use, I'd bet on the PM or WM.
My first "police" shotgun came to me built up on a Wingmaster receiver with orange furniture, indicating it was used as a "less-lethal" munition platform. Ugly furniture, but the receiver was oh-so-slick.
I keep my eye out for used PMs and WMs. I have come to appreciate non-ribbed bbls and fixed chokes, so there are lots of old WMs out there to be had for the price of a new Express or even less.
FTR, I don't have anything AGAINST the 870 Express line. I just prefer the PMs and WMs for their particular virtues. Neither WM/PM of mine is pretty. Both are rigged functionally for their tasks.