Not sure. I know when I met my OH there was an instant immediate attraction and our phone bills immediately went through the roof. I also knew he would irritate the arse off me, which he does............ It wasn't instant but we did fall in love very quickly.
Met my second wife at my wedding to my first wife ( they're second cousins and couldn't be more unalike) and I knew there and then I shouldn't get married but did anyway. Took a long time but finally life conspired for us to be together and although we're apart now she's still the only woman I've ever felt anything like that about, so yes I believe in love at first sight.
What's to be confused about? It's ok to believe what you want, if you want to believe in love at first sight then you crack on with that, it'd be boring if we all thought the same (not to mention terribly unromantic if everyone thought the same as me!) :c)