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Were you using a 3/8" drive ft-lb range torque wrench? Those are going to be quite inaccurate at the lower end of their torque range.
16.5 ftlb is kinda borderline but I'm guessing that was your problem.
For low-torque bolts/fittings you're going to want to use a 1/4" drive in-lb torque wrench, they usually go from 20-250inlb or something like that, so you'd be using it around 200in-lb in the meat of it's usable range.