Remington's LE 870s with synthetic furniture have short (7") forearms too.
The "field length" forearms (usually 9") give more to hang onto, and work better for shorter statured shooters when used with appropriately short LOP stocks. If the support hand doesn't have to reach out so far to get a grip on the forearm, the gun is easier to support and swing.
Most of the aftermarket furniture for 870s is aimed at the "tactical" market and thus looks like the LE factory stuff. Not covering the loading port with the action open allows easier downloading of the magazine tube by using the shell latches to slip rounds out of the magazine without having to run them through the action. It also allows using the shooting hand thumb as a magazine cutoff to clear the chamber without feeding another round onto the lifter.
fwiw,
lpl