From what I found in a HP article on the Web 1/6 in 35 - around 0.02% - is comparable with the stability of most printing media so it could be a change in temperature or humidity that caused the problem. If a fair amount of time (a few hours perhaps) passed between printing and checking there is no way to be sure whether the problem is due to dimensional drift or printer error.

You and Bill hit the nail on the head. The media will change depending on temperature and humidity (like all materials).

Engineering prints are not spec'd for use on direct work. They are not templates. Especially in this day of CNC work. Traditionally you aren't supposed to to even scale off them.

If you wish to use a print as a template consider having them print on Mylar. It's more stable.