In a nutshell, English spelling has been fluid over the years and no doubt will continue to be so. The book I am currently reading — 'Perdita" is set in the time of George III. Spellings favour -ize rather than -ise (a later change to the French -ise occurred in Victorian times). The Georgians (and those before) were idiosyncratic in their spelling, so I suppose if they were happy with variants, then we should be too.
Take a look at the ‘Pants in the boot’ podcast for a humerous approach to fractured English!
Brit here, who spends a lot of work time polishing EU English. I’m not knocking Euro-speak – my own foreign-language skills are distinctly lacking – but it truly is a language of its own.