You should be able to return or exchange your HF compression gauge. You must have the receipt though.
You have to tead the instructions! It tells you to use the supplied copper washer if you use the inj adapter.
The injector adapter will give you a lower reading than the glow plug adapter due to the extra volume in the inj adapter, provided both are sealing well. Absolute accuracy is less important than relative accuracy in a compression gauge.

I've found that there are too many poor quality fittings in the HF set, each being a potential leak source, that I have modified mine with my own quick fittings and hose, so that it is quick/ easy to use and accurate. I can take a compression reading in less than 2 minutes per cyl via the glow plug holes of a 617 except #5, which takes longer because of tight access.

__________________
85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked