I've dismantled and rebuilt a lot of Briggs over the years, I've never ran into one that didn't, but I rarely rebuild any that less than 10 years old. Heres the official line from from briggs.
All engine models named Vanguard, I/C, Industrial Plus, Diamond Plus, Intek Edge, Intek Pro, ELS (Extended Life Series), 3LC (Three Cylinder Liquid Cooled), Animal or Europa, have a cast iron sleeve in the cylinder bore. All other engines are dependant on the model, type, and code numbers to determine whether or not the engine has a cast iron sleeve installed.
I had always assumed it was a marketing ploy. I know for a fact that horsepower's are often under reported. They do this to show slightly different models and prices. I will say that if you match the two engines up side by side and they look the same and the smaller engines vary by some fraction or the larger ones vary by one or two horsepower they are the same engine.
However I guess I was wrong, it didn't used to be this way. I would guess they just started matching up life expectancies of the motors with the equipment. Some of the low end roto-tillers only have a design life of 120 hours. I would imagine an aluminum would only last that long.
Will