I think, and I could be wrong, but the extra weight on the LE might be from the 2 coats of slow cure finish on the blank. The LU has matte green blanks with finish on the guide wraps only. Might be a difference in the strippers or the reelseat also. Its not enough difference that I would probably notice though.
I was comparing the St. croix LU, and LE series to the G-loomis GLX and the Scott STS and they are all very close in weight, but the latter two are more pricey and no one around here carries them.
I have owned 3 different LU series rods, a 3wt, 4wt, and a 5wt. They have all been good rods and SC has always given me good warranty service.