Wikitionary claims that leftenant is an archaic spelling of lieutenant. It's an achaic spelling in English, but not in French. This spelling was to stick to the pronunciation, and not the opposite, as there is not "lefttenant" in old French.
If you're not up to speed with your old-colonial vernacular, here's the deal: Canadian English dictates the word “lieutenant” be pronounced lefttenant, rather than lootenant. Linguists explain that the “f” is a relic of the country's British imperial history, while the alternate pronunciation comes from the U.S.