Originally Posted by Hardyreels
I think it has to do with the various head / belly lengths and the fact that many Spey - Skagit & Scandi lines far outweigh AFTMA fly lines which are used with traditional fly rods. I understand that you are frustrated trying to decipher the Spey code but trust me, in time most of it, the part which pertains to you will make sense.
+1 to what Ard's said in his last two postings. The primary reason for using a spey rod is you're fishing in tight quarters (tree line/brush right behind you) where you have limited room for a back cast with a single hander.
Second is an all day session with a single hander will/can really sap you dry from fatigue. With a 2hander, if you're properly casting, you won't get any where near that bit unless you're using 15 foot and plus heavy rods.
As to 'why' all the different lines? Depends where/how you want to fish (and to a degree for 'what') will dictate what line(s) you will use. The short heads are 'mass in the ass' to chuck heavy wet flies, sink tips, etc.
As to why these are rated in 'grains' rather than the AFTMA scale that has to do with AFTMA only considers the first 30 foot of line. A spey line rating (in grains) takes into account the entire head regardless
of length. Or another way of putting that is the line is 600 grains, but that could be in a 35' head or over 65 feet.
With rare exception (Gary Anderson's custom rods comes to mind) most 2handers have a 'grain range' that will properly load a given rod blank. You're choice (depending upon your ability as a caster) is to choose something within that range that properly loads the rod for your casting/fly of choice.