"Center" is the American variant spelling of the British "centre"

Correct. Much of the 'bastardised' British English can be attributed to Noah Webster, (Merriam Webster), and whilst I highly respect M-W I have never been able to understand why, in many instances, Noah Webster only did a 'half-job'.

I mean, if you are going to CHANGE the spelling of a word such as 'centre' into 'center', why no go 'all-the-way' and make it 'senter'?. After all, that's how it's pronounced, lol.