If you read the stuff at the links, it explains that in Old French "lieu" was often written as "luef". So faced with "lueftenant" it seems more understandable how "leftenant" might arise.
"Lieu" on the other hand as a standalone word might more naturally take its pronounciation from the modern "lieu".
The US and Australia may have adopted the other pronounciation for historic reasons, or maybe even just to be different, which might also explain the interesting inter-service discrepancy New Judge has pointed out.