Johncke has it nailed! Just because Jerry Seim, or Jimmy Green for that matter, liked a 3wt line on a particular rod (such as an old 390-4 RP rod like the one I have) does not mean it is carved in stone. It is merely a recommendation based upon the rod designer's personal preferences.
In the old days, before every aspect of everything being so over regulated to the point it is today, tournament distance casters made up their own shooting heads by chopping a high density DT line of two, or three sizes under the rods designated weight. The resulting head made up for the needed grains by adding length, and was much more aerodynamic than a short fat thirty foot floating head. The real kicker though was they had discovered that a fly line will only continue to fly until it has completely turned over. At that point, it runs out of energy & starts to fall. These longer heads, with custom spliced tapers, delayed the turnover as long as possible. So much for ancient history.
Many line manufacturers have marketed lines that deviate somewhat from the ACA standards, halfway between sizes, long bellies, or extra long rear tapers. A lot of saltwater schools used to recommend their clients over line rods one size and chop the tip back. No big deal. These & other variations have been going on for decades. Not only in the single hand world, but the two hand world as well! Alexander Grant made his own lines over 150 years ago.
Well, perhaps I'm getting carried away here and straying off topic. But I remember (that's what happens when the years start stacking up on you
) reading about Charley Waterman teaching a beginning fly caster to feel the rod loading by stringing a 10 or 12 wt line on a little three weight rod. When the student got to the point he could feel the rod loading, Charley would line the rod lighter. It was an exercise, a tool used in the teaching process to drive home a point. Later, perhaps days or weeks, by the time Charlie got that student down to a properly lined outfit, they were taking a break when Charlie' wife Debbie came out to go shopping. She picked up the rod, made one false cast and then said "oh looky, I shot the whole line"
I've heard people say "those who can do, those who can't teach" I'm not so sure of that anymore. I've come to the conclusion that just being a master at something does not automatically qualify one as a good teacher. Teaching is a skill in itself.