Rod manufacturers design the rods to perform best for the use the rod is likely to be put, even within the same series. The way the typical person would use a 3 wt is different from how they will use a 6 wt or and 8wt. In a 3 wt, presentation is usually key, so the action will likely be slower. Once you get to 6 wts and above, distance and butt strength are more important, thus, you tend to get a faster action. This is also why you see comments that upper weight rods are "under rated", because people have to overline them to get close to the same action as the lower weight rods.
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