Sounds like the British guy who discovered it settled on the spelling without the extra i
A January 1811 summary of one of Davy’s lectures at the Royal Society mentioned the name aluminium as a possibility. The next year, Davy published a chemistry textbook in which he used the spelling aluminum.
https://en.wikipedia.org/wiki/Aluminium The Americans have one less «i», not the other way around.
Sounds like the British guy who discovered it settled on the spelling without the extra i