Normally the rail extension is not the problem, no matter who the rail extension is made by. Most rail extensions (including ours) are machined on a CNC mill. Which means they can be duplicated with very repeatable tolerances.
Tracks on the other hand do not hold as tight a tolerance as machining aluminum. Most track manufacturers have about a +/- 1% tolerance on the actual length of the track. Which means if a track is short to the extreme end of the tolerance (136*0.99) the actual track length could be 134.6" ... that's 1.4" short! You wouldn't think that makes a big difference, but 1.4" short has a large effect on the amount of adjustment range for the tensioner.
Typically the extremes of the tolerance are found more commonly on tracks purchased in the aftermarket. The sled manufacturers are more stringent on what they receive from their track vendors.
I myself ran into this same problem a few years ago when the tensioner was all the way loose, and I could barely even get the track installed. I measured the rail extension, and it was spot on. However, I also measured the track, and it measured at 134" ... 2" short.