Stuck in centimetres, though preferences set for pixels

I'm on Mac OS Sierra running the latest 2017 Photoshop. I'm desperate to get the image resize dialogue box to default to pixels but it keeps showing centimetres as a default. I have changed this in every previous version of Photoshop I've used, and it's always worked the same way, by changing the units in my preferences. For this version I've done it multiple times, closing photoshop and reopening, and have restarted my mac, but I still see centimetres for the width and height inputs.

Is this a bug or am I missing something? This is seriously frustrating my workflow!