Just had a similar experience for the RV8 DWG 11A
And then there's DWG 38, which shows you how to trim the rear spar doubler for drilling to the fuselage. DWG 38, however, doesn't come with the wing kit (or at least it didn't), it comes with the fuselage kit (a good reason, btw to have the preview plans on hand).
DWG 11A is the center spar section drawing for the RV8, which provides the location and proper orientation of a couple of AN4 nut plates that go on the root of each main wing spar. Their purpose, however, is to accomodate some bolts that attach the wing spar to the center section (which you don't get with the Wing kit, but with the fuselage kit for the 8). The instructions, however, did not seem to mention the rivet callout for these nut plates. So of course, those will be listed in my large DWG 11A drawing, right? WRONG!
Since the reference to DWG 11A was in the instructions for my Wing spars, I surfed through my large Wing kit drawings for DWG 11A - not there. Finally figured out that I don't have the large plan for DWG 11A cuz the center section for the 8 comes with the fuse kit, which I don't have yet.
So I had to resort to the smaller DWG 11A drawing in the preview plans to find the rivet callout that I was looking for.
Anyway, to answer the original question about the piano wire for the trim tab hinge - I think Vans intention is to give you enough to actually make and fit the trim tab to the elevator, realizing that this will be installed and removed perhaps several times over the course of the build while you fit it to the elevator, fit the empennage to the fuselage, or for painting, etc. So they save the correct sized pin for later. Think of it as using the first one for building and trial fitting where it can get bent up, scratched up, nicked, etc., and then they give you a brand new shiny one later on down the road.
Given recent incidents that seem to involve trim tab assemblies to one degree or another, I guess I kind of like the fact that I get a new pin a little later on in the build. Just wish they would not give you instructions in the preview plans that first tell you to install it, followed immediately by - "you wont be able to install it because the pin provided is too short." Seems kinda dumb to me to that they do that.
So NO, you are not crazy - just dealing with some badly written instructions from time to time like all the rest of us.