Just had a bit of frustration printing an A5 portrait page (but solved it!)

In Page Options, Page Size, I choose A5.

When printing, I select my printer and set to A5, then the DP+'s print dialogue says Automatic Page Fit not okay.

I'm trying to understand this. So I check what size an A5 page actually is.
It's 148mm x 210mm.
BUT, one website says (amazingly!) that 148.5mm is okay too!

Back into Page Options, Page Size, DP+'s greyed out dimensions for A5 are 148.5mm x 210mm.
I check my printer and it says A5 is 148mm x 210mm.
Hence the 'Automatic Page Fit not okay'.

Back into Page Options, Page Size, I choose Custom and make the size 148mm x 210mm.
Back in the Print dialogue, it now says 'Automatic Page Fit okay'.

Back into Page Options, Page Size, and the size name 'Custom' has disappeared.
Now it says A5, and the greyed out dimensions are 148mm x 210mm!!

Can we make A5 standard at 148mm x 210mm?

Seamus