So the difference, for me at any rate, is when there's no reason for the man to be condescending except that the subject in question is something that men are by default, according to society, expected to know better, and he doesn't engage in that sort of behaviour when confronted with a man. Bonus points if that's a subject that the woman is actually extremely well-versed in, and the man doesn't actually check to see how much she knows before launching into it.

So if John treats everyone like a five year old, he's a condescending asshole.

If John only treats women like they're five, but all the time, he's probably a sexist condescending asshole.

If John tries to explain the anesthesiologist pay scale and work environment to an anesthesiologist who happens to be female, but he himself doesn't have the medical, business, economics, or public policy background (thus rendering him generally incompetent to lecture anyone about the subject), and there's really no other reason why he should think he knows any of those subjects better than her, he's probably mansplaining.

I think it's subtle, but there's definitely a gendered component to some forms of condescension. Usually with a dash of societal expectations. Very rarely malice. Women who work in more traditionally male, or heavily male dominated fields probably experience it a lot more than others.

There's an essay by Rebecca Solnit called "Men Explain Things To Me" that explores the phenomenon pretty well. The anecdote that sparked it is about a man she meets at a party who tries to expound on a certain topic, even mentioning a very important book on it that recently came out, without realizing that she's the author of the book. Also he hadn't even read the book in question. And her friend had to tell him 3-4 times that she actually wrote that book before the man realized.