[MrForbes]

Look at the top right corner of the first sheet, it has the revision history. It states that revision 1.1 was to shorten the length of item 5.

My guess is that they forgot to change the length in the parts list, but they did change the length on all the drawings. Since it's a scale drawing, you can see that the part is really only 36" long.

You might want to ask on the official Q&A though.

If you make it too long, you can cut it off later. If you make it too short, you can glue on a coupler and another piece of pipe later.