A 1' (25.4) steerer fork will take a 22.2 stem also known as 1" stem.
A 1 1/8 Steerer fork will take a 25.4 stem also known as a 1 1/8 stem.
Hope this helps
In your case, your stem has a shim installed to compensate for the size.
A 22.2 stem + shim = 25.4 to fit into your 1 1/8 fork.
Thank you very much
You have cleared up my confusion regarding fork steerers and stems.
I thought that was the case too.. but the shim doesn't go into the fork steerer it sits on top of it inside the top half of the headset.
I bought a 25.4mm quill for it but it wouldn't fit.
I have now bought a 22.2mm quill and it fits perfectly into the steerer, but here is a slight gap at the top of the headset where the shim was on the last quill.