Why does the Margin Move

Just a quick question
Why does the margin move
Like for example I’ve put £128 on Nio on CFD today and when I was on +60
My margin was £128.27
But now when im on +30 my margin is £126.7
I’m not sure I get it
Now Nio is on the rise my margin is also going up now it’s £127.3
Not really a problem just confusing Confusing

I think it’s something to do with exchange rates on overseas stocks

Its happend before It’s for every stock I buy on CFD even ones from my own country