Any help on this one is much appreciated.

At the moment I've got Schwalbe Marathon Plus tyres which are marked 25-559 26" X 1.

I wanted solid tyres (had too many punctures when travelling alone) so I bought some Shox G2 which are 20-559.

I would have expected them to be the same size, 559 diameter. But the Shox fits inside the Schwalbe tyre.
When I look at the marking on the Schwalbe it says 25" 20-559.

I don't understand, how can one be 26" and the other be 25"?