Prop pitch is correctly determined for your specific boat/motor and how you load and run it.
Every outboard has a "WOT range" (wide open throttle range) where the motor was "designed" to run wide open.
It is usually above 4500 rpms but below 6000 rpms - again, depending on the exact motor. (sorry don't remember what the WOT range was on this model)
If you are running less than maximum horsepower for your boat and/or you tend to run heavy (lots of gear, passengers etc), the motor will have to work harder so the rpms will be lower than if you ran light etc.
Reducing the pitch will allow the rpms to increase (all else being equal), increasing the pitch will reduce the rpms as the motor works harder or easier to turn the prop.
For example, if you are currently running say a 15 pitch prop and your wot range was 5000-5500 and you were only making 4800 rpms full tilt that would mean you are running "too much prop" (too much pitch).
Rule of thumb - reduce or increase the pitch by 1" will result in an increase/decrease of 200 rpms. So if the 15" pitch was "too much" (4800 rpms), moving down to a 14" pitch should increase the rpms to 5000 (minimum wot range). In that case perhaps moving to 13" (+ 400 rpms) would be better - give you some wiggle room.
Other changes to the prop can also effect rpms. Increasing or decreasing the diameter (if available) will also have the same effect - about 200 rpms per inch.
Going with cupped blades normally drops the rpms by 50. Going from 3 to 4 blades will drop it another 50-100 rpms (with the reverse true).
Stainless props are less (flexible) so bite the water a little harder than aluminum (which bites a little harder than composite/plastic), so those will also minimally effect the rpms.
And finally, the time of year/weather/humidity/air temperature will effect how well the motor can "breathe" and that also effects the rpms. The motor doesn't work as hard in cooler/drier air so if you prop it to just barely into the WOT range say in the spring it might be gasping for air and not making the WOT range in the middle of the summer when it's hot/humid.
The lower the horsepower the less the effects will be noticeable (plus there may be less prop options) so typically you shoot for the middle of your WOT range and that will generally work "well enough" under various conditions.
So it might sound a little complicated but I only answered this way to illustrate that choosing a prob is not as simple as looking at some basic chart that says 15 foot boat with 40 horse motor uses X prop - or just because Bubba turns a 16 pitch with the same boat doesn't mean it would suit how you use/load your boat.