Liverpool have been superb and by far the best team in the league. That much is clear. Perhaps saying "lucky" is being disingenuous, but the luck has undoubtedly been on their side.
Virtually no injuries (even City have been screwed by their CB injuries - if Liverpool had the same injuries as City or United they wouldn't be unbeaten for sure). Opponents missing sitters against them. Lots of those last minute victories from very fortunate errors.
Of cause the latter is born out of supreme fitness amongst other things, but in a different year they don't all go your way.