Can't stop double-sided print from down-scaling

I have a number of PDF files that I need to print off at 100%. Some are single-sided; some, I need to print double-sided.

If I select "Actual size" in the print dialogue, all sheets are down-scaled by about 10%.

I managed to get around this for single-sided print by selecting "Custom Scale" instead, and specifying 100%. On that setting, single-sided prints come out correctly. However, that doesn't make any differnce for double-sided printing - as soon as I then select "Print on both sides of paper" as well (leaving all other settings untouched) the problem returns, and both sides of the print are once more downscaled.

I've been through the print dialogues to the best of my ability looking for any other setting that might be changing when I select "both sides"; I can't see anything. Nor can I see anything in my printer's setting that might be affecting it.

Can anyone help? Right now I'm having to go through printing double-sided documents one sheet at a time.

I'm on Win 10, and Adobe Acrobat Reader DC "Continuous Release v 2018.011.20038".

My printer is an HP Photosmart 7520; I've checked that my drivers are up to date.