The 1100 and 11-87 are 99.9% the same. Field grade 11-87s have a gas relief valve in the barrel ring and can shoot anything from 1-1/8 ounce field loads up thru 3" Magnums. 1100s are 2-3/4" or 3" guns with the 3" guns often will not shoot lighter loads. Most current guns the 11-87s are dull and aimed at the hunting market, whereas the 1100s are polished for the Target set. The 11-87 also started with a wider extractor, but the newer 1100s have that too.
What could have happened... did.