The camshaft, which is really the 'brain' of the engine, and carburetion make the difference. Also creative labeling in some cases.

I know looking at the Briggs IPL's, sometimes the only difference is the decal. Often the carb is a little bit bigger/smaller.
I haven't seen a source for Kohler IPL's, but I would assume the same to be true.
The fact your 18 horse has SLIGHTLY better torque spec's than the 20, I'd assume carb. The smaller carb mixes the air/fuel better in the lower RPM range, than a larger carb.

Interesting. So from the manufacurer's point of view there is no cost difference in making the 18 or the 20, the engines are identical, the slight difference in cam or carb or both are just differences, they will cost the same to manufacture. Smart way to keep costs down.

I was thinking that along with max torque they ought to show torque @ 3,600 rpm so we'd know the full throttle torque figure. If the torque curve fell off a lot at high rpm's I'd think the power to mow high grass or operate a snowblower would suffer.

Some engines are now displaying the maximum torque ratings but most folks do not understand what they mean.

From a "marketing angle", the OEM of any product can utilize "creative" labeling or titling (naming, or describing the product) as an alternative to actually making improvements to a product. The company I worked for in the 1970's produced an engine for medium duty trucks. It was a V-8 Diesel. The engine was prone to lots of "small problems" (oil leakage, heavy smoke, difficult starting and some other things). The engine was introduced as "DV 550" (Diesel, V-8, 550 C.I.D.). After a few years on the market, the consumer became all too familiar with the "problems" and sales of the engine declined. Next, the same engine was renamed as a D-??? (insert applicable horsepower to fit specific rating chosen as it was offered in 3 choices of HP (example...D-190 ). Sales rose (somewhat). After a couple of years, sales sagged again (as people realized that this was the same engine). The last "title" bestowed on this engine (to my knowledge) was "9 Liter". Once again sales bounced up slightly only to fall again after the consumer found disfavor with the product. The point here is that with a lack of "Industry Standard Nomenclature" rules to cover how an OEM presents the product in the market, it can be called, or described as "whatever sounds attractive" to the potential buyer. Most of the time there are no "Bold face lies" in the labeling, you just have to peek under the camouflage and then comprehend what you see. Often (in corporate affairs), the "Sales Engineer" is the most highly revered of all "engineers".

"Interesting. So from the manufacurer's point of view there is no cost difference in making the 18 or the 20..."

Well, not always. For example the difference between the 18 & 20 HP Vanguards is that the 20 HP has a "2 barrel" carb. I would guess it cost a couple $ more, since it has 2 jets etc. That might cost $10!:-)

On some outboard motors the difference is there is a restricting plate behind the carb that limits the flow into the block, thus reducing horsepower. A common example of this is the 9.9/15 hp motors. If you look, some of them are exactly the same. Just a thought.

The difference between the ratings shown is simply RPM.
HP = T*N/5252
44.4@2200 = 18.59HP
44.2@2600 = 21.88HP
Since torque is nearly identical you gain hp by moving the same load further in a given time... RPM gain does that for you.

Calculating Costs(Cost of Manufacture) is a Science.
Developing Prices(Selling Price to the consumer) is a Art.
There is very little direct relationship between the two. Hopefully the difference(margin) between the two is enough to sustain the product line or better yet the company.