It drives me crazy

, i've always spelt realise with an s and it wants me to swap the s for a z, so i checked the Oxford dictionary and yes they also spell it with an z, however i googled it and it's American to use a z, so then why is it in the Oxford dictionary

it's the same with organise, i've always used an s and it wants me to use a z.
I thought i was good at spelling and this has shaken my confidence
Anyone else had a confusing/confidence knock with spell check?