The basic difference is that the 1187 uses a pressure compensation system so that both light 2-3/4" and 3" magnum shells will cycle in the same barrel.
The 1100 requires two different barrels to accomplish this, because the standard 2-3/4" barrels and the 3" magnum barrels have different gas ports.
If you look at the "barrel hanger" (properly called the gas chamber) of an 1187, you'll see that it has two internal gas ports going into the barrel. Then there are two pop off ports on each side of the 'barrel hanger' with a flat band spring over them. When light loads are shot, the spring closes these pop off ports. When magnum loads are shot, excess gas is bled from these ports.
Note that not all 1187 barrels have the gas compensation system. The exception are special barrels. Trap and skeet barrels are chambered for 2-3/4" shells and these barrels cannot be used with anything more than target loads. On the other end of the spectrum are the 18" defense barrels and the 21" slug and turkey barrels, chambered for magnum loads and slugs. These barrels will not cycle lighter loads.