Fly rods can and do cast different weights because we cast different distances each cast. I often cast a 7wt line on my fast six weight rod bass fishing from my kayak because almost all of my casts are under 30'. I cast the same 7wt line on my fast 8wt rod when i am heaving 75+ foot casts bass fishing from the bank.
Manufactures try and hit a happy medium to and base the rods line weight on 30' of line out the rod tip. The rods line weight is base on a company expert's opinion casting 30' of line,your opinion my vary.