As a spelling mistake? Or perhaps they chose the US spelling as the company (Optimize Services Ltd) who registered it has the US spelling, and not the correct spelling, and wanted their domain to match?
The same as mom.co.uk and aluminum.co.uk, just because the domains exist doesn't mean they're the correct spellings.
If I had registered I would have got both- I expect this sort of typo will get more and more common as the Americanisation of the Interweb continues...