There are actually more slots than that. The hip bow slot is not the same as the sword slot, you can have a nomad/khergit/strong bow and a sword visible at the same time. There are three back slots, bows and crossbows go over the left shoulder, while two handed swords and arrows go over the right shoulder, and shields go in the center.. There's also a dagger slot on your belt that's used by daggers, knives, and the wakizashi.
There should be enough weapon slots in the inventory screen to allow you to fill all the model slots, as it makes no sense for example to not be able to carry a dagger just because you have a bow, a sword, and a pair of quivers, and you should only be able to equip one item into each model slot. Lastly, there should be an "in-hand" slot for polearms and possibly other large weapons that you can't carry on your person. You can carry these in your hands, but it doesn't really make much sense to be able to stroll around with a pike tucked in your pocket. If you want to switch to another weapon, you'll just have to drop the polearm.