B & S Sell their engines by capacity, the first 2 numbers are the cubes.
The stickers on the engine are specified by the mower maker.
Generally a new engine is choked back to give the required power output.
SO in many cases the difference between a 26 & 30 Hp engine will be the manifold, a gasket and the governor spring.
Simple things so they can supply 500 x 26 Hp engines to one company and 300 x 27 Hp to another and 400 x 28 Hp to a third without making major changes.
The wholesale price of the engines within a capacity are basically the same so it is not uncommon to find a 30 hp engine wearing a 26 Hp sticker cause Briggs had 1000 30Hps hanging around from a previous order.
If you really want to know what is different then download the parts book and check .
In the description section will be the different part numbers for things that are different and the serial or code numbers they apply to.
Contry to popular belief no engine maker makes different mower engines in 1/2 Hp incriments from 10 to 40 Hp.
As for slippery salesmen, walk around and find a 30 Hp briggs engine and check the model number.
If it is the same as your 26 then there might be some truth to it
