Hi R, I really don't know if there is a definite answer to your question, but here is my opinion...
I do think that there can be a difference. Some people appear to be quite heavy drinkers, then they just decide to cut it out, or cut down or whatever, and the appear to manage to do that AND to keep to the new lower level without too mych apparent difficulty. I say 'apparently' because it is impossible to know what it going on in someone else's head, and there may have been times in my life when I looked as though i was cutting down without a problem whereas in reality I was struggling terribly (and it never lasted).
I do think that some people have got into a habit of opening a bottle of wine, in the same way as some people always have a cup of tea when they get home (no matter how late it is) or get into the habit of surfing mumsnet when they are bored. I do still think that changing a habit can be tricky, but it feels a little arkward rather than feeling like mental anguish.
I do not think I fall into that category. When I try to cut down my drinking, I am tormented by it - counting units, working out which days I can or cannot drink, regularly drinking more than i had planned to. That is much more than just adjusting to a new habit I think. I would say that I am an alcoholic. Even when i am not drinking at all I am often thinking about drinking (or about the benefits of not drinking). My sister who is not a problem drinker, does not appear to have those thoughts running through her mind.
If you think you have a problem, then why not experiment (you do not have to be right first time). Try cutting down. Buy a really good bottle and drink ONE glass. If you feel like it have ONE more glass the next night. Then pour the rest away. Notice how you feel. Are you worrying about the waste of money? Really? So why did you not buy a half bottle. Or are you struggling because you want more alcohol? Do you only drink too much when you are bored on your own? If a friend came round would you have a cup of tea and a piece of cake instead, or would you ask them to bring some wine so you can get pissed together.
I can't tell you the answers to any of those questions, and you may not know either. So try it out. Are you drinking tonight? Can you stop? Some on here find it easier than others.