Post Chosen, Buffy does a wish or somesuch and gets flung back to 1900, landing in Peking, just in time to see Spike get his first Slayer.

She knows objectively she should aid Xin Rong because this Spike is still evil, but she can't bring herself to help dust him, because (a) she loves him and (b) if he dusts now, what happens in her time? And then the other Slayer sees this white girl standing and observing, and assumes she must be a vampire too, and splitting her attentions like that makes Spike's job easier. And then Buffy is whisked away.

And then she's in America again, but it's way too early to be the 70s. Because this is a fic where Spike did go to fight some other Slayers in between, but didn't actually fight them because they weren't up to his standards. But Buffy doesn't know that yet. 

So she finds the local Slayer and helps out while trying not to muck up the timeline too much, but when Spike makes his move, the Slayer still isn't strong enough to give him the fight he wants, and he disappears into the night. And then Buffy disappears too.

This happens a few more times. And Spike figures out that this weird girl who keeps appearing every time he investigates a Slayer must have something to do with either him or with Slayers, and he's curious, especially after he tries fighting her and she's strong enough to dust him but she doesn't.

And that means when Buffy gets to 1977 NYC, Spike isn't actually interested in killing Nikki Wood. He wants to solve the mystery girl. And Buffy realizes her timeline is well and truly beyond repair.