I didn't (mean to) say there is a definit fit for each rod in general.
My experience is, that all fly fishermen in my lessons ended up choosing the same head weight on a specific rod in a specific situation using a specific fly to be cast.
And that leads me to believe there is an avergae range of rod bend (during casting) that fits (feels) best for a high percentage of fly fishermen. And I think that those recommendations we all give are partially based on that (as on the single circumstances of course).
Again besides all this, a knowlegable caster will always be able to adjust his movements to each rod-line combination, even if it is extreme.
A good example to me is Tarpon fishing. I need a 10 or 11 wt. fly line in order to (comfortable) cast the flies I want to present to the fish. Strength of those lines are excellent. The splash (due to fly line weight) is perfect. Using a 13 wt. fly line would splash away lots of fish. That heavy line is too hard to present splash free close to the fish.
Using an ERN 10 to 11 fly rod forces me into highest necessary concentration during the fight. The smallest mistake and the rod is broken on these large fish. Using a 13 wt. rod gives me much more space here. I feel much relaxter during the (long) fight.
But then again the ERN 13-14 fly rod bends much less during casting when using a 10 or 11 wt. fly line. Sure I can handle that - especially since it is only a few chances to present my fly during the whole day anyway. But casting a 10wt. fly line on an ERN 14 fly rod feels uncomfortable to me. I feel significant more comfortable when using a 10wt. fly rod here.
So I decide for a compromise in using a 11wt. fly line and a 12wt. rod. If you want, I "underline" about one line weight.
Same thing happens when casting the 5wt. MED on the HT. There are stiffer rods (like my 690 SP+) that offer me a longer carry. But casting the whole spectrum of different line lengths from short to long I feel best with the HT. The range of rod bend during the whole spectrum of line length matches perfect for me.
In that kind of feeling for a matching rod-line combination (based on the range of resulting rod bend) I still think a high percentage of fly fishermen are relatively close together. That simply is the result of casting and experimenting with a huge amount of other fly fishermen trying different combinations. And that is also the only reason I can think of why those shooting head recommendations - like the one RIO offers - work fine for many fly fishermen.
Most of my line recommendations to students are based on the average situation since I hardly ever know in what kind of different situations my students may use their rod-line combis in future time. So I make a recommendation based on the feel for a certain range of rod bend I think they will produce by a specific combination of line weight (in relation to length etc.) and rod stiffness in the average situation.
I agree it always can only be a recommendation. But I believe it to be a very good one mostly.
Does that make more sense to you?