As an American, I was never shocked to see the word "center" spelled as "centre." It didn't bother me at all. Honestly.
But then I saw the participle of it spelled as "centring" as opposed to "centering," and it really made me reconsider the whole thing. Do UK (and Australian, New Zealand, etc.) speakers pronounce it similarly to US speakers (three syllables)? Do you guys ever spell it "centering?" Do you find the US spelling as weird as we find the UK spelling? Does it make you uncomfortable in a way you can't fully understand?
Also, I see that my Chrome spell check is equally thrown off by the UK spelling. Is your browser's spell check disgusted by the US spelling?