Funny stuff.
In one thread we'll tell a guy that a rod will cast a variety of line weights, usually one full weight in either direction. In the next thread we'll fuss over lines being a 1/2 a weight heavier, or whatever.
Hehe, it's just marketing. Works like this. You choose your rod based on test casting at the shop. Distance is king. If your testing a rod you're bombing casts out there, not how it feels at 20 ft. And you're gonna buy the one that feels best when you're really opening up.
But lines, you're not testing like that. You have a line on there, and think it's fine. Someday it wears out and you have to replace it. If you think the replacement feels better, you go and rave about it on a message board and spread the word on what an improvement this new line is.
Now, back to rods. Take a bunch of 5 wt rods out to the lawn with a reel lined with 5 wt line. Have one 6 wt in the bunch (which will be underlined). When you're bombing casts at the top of your range, guess which one is gonna feel best?
"But I don't want a 6 wt, I want a 5 wt!"
Fine. A smart rod manufacturer says "hey, if we just make a 6 wt, and label it a 5 wt, it'll sell better! We'll call it fast action." And that's just what they do, and at the fly shop, this rod wins the lawn test and we all hand over the bills.
But now you're on the stream, and this rod, in all actuality a 6 wt, is labeled a 5 wt and you have it underlined with 5 wt line. It may be great when distance casting, but it's sub-par in close where you actually do your fishing. Some old timer tells you to put a 6 wt line on there.
"I can't do that, that's overlining, and it feels wrong. I think I'll trust the manufacturer."
So a brilliant line manufacturer takes a 6 wt line and labels it a 5 wt! Throws on some marketing gold like "designed specifically for modern fast action rods", which, translated from marketing to English, means "weight is under-rated to match today's under-rated rods".
You go to a stream, and what do you know? This line feels better on this rod. Must be something to this "designed to match modern fast action rods" thing! Ain't technology great?
Yesterday's 6 wt has become today's 5 wt.