I suspect some libertarians believe life begins at conception, and therefore advocate illegalizing abortion. Some believe life begins later, and would allow abortion. You'll find the same in the Republican Party. To a much lesser extent, also the same in the Democrat Party.
You are confusing politics with ideology. Although I have previously pointed out that Harry Brown is the political leader of Libertarians, he is also true to Libertarianism, unlike many "Conservatives" who lead the Republican party. If one is true to his/her Libertarian beliefs, then they must be for keeping Abortion legal, because Libertarianism is a Utopian political dream.
I once thought that I was a Libertarian, but my objection to Abortion keeps me from making that claim.