I think it all depends on what one's expectation is by "world's best airline" (or world's best anything, really).
Does it mean the world's best airline must be the best in ALL aspects? Clearly that is an unreasonable expectation.
Of course other airlines do some things better than SQ does, but they also do other things worse than SQ, so on an overall basis I suppose the majority of travellers / voters feel that SQ does most things better than all other airlines. Which is good enough to grant it the "world's best airline" title.