Mike,
You are spot on with that. I have a Bloke XL50 which when you take it out the tube, feels incredibly light however, stick a line through it and cast and I find it heavy and quite frankly, dull. (Its the 7 weight) I have spoken to a few XL50 owners about this and it seems that the 7 weight has a bit of a reputation for this but the lighter rods in the range (as in line weight) feel lighter when casting, as I found out when I borrowed Iains one at Glentaner.
I also had two of my 9 weights built up, one with a Struble Saltwater Titanium reel seat which is very heavy, the other with a standard fuji reel seat, very light.
The rod with the heavy reel seat felt so light to cast, the other felt tip heavy. Identical blanks and identical fittings except the seat.