AVAssetWriterInputPixelBufferAdaptor and CMTime

I'm writing some frames to video with AVAssetWriterInputPixelBufferAdaptor, and the behavior w.r.t. time isn't what I'd expect.

If I write just one frame:

 [videoWriter startSessionAtSourceTime:kCMTimeZero];
 [adaptor appendPixelBuffer:pxBuffer withPresentationTime:kCMTimeZero];

this gets me a video of length zero, which is what I expect.

But if I go on to add a second frame:

 // 3000/600 = 5 sec, right?
 CMTime nextFrame = CMTimeMake(3000, 600); 
 [adaptor appendPixelBuffer:pxBuffer withPresentationTime:nextFrame];

I get ten seconds of video, where I'm expecting five.

What's going on here? Does withPresentationTime somehow set both the start of the frame and the duration?

Note that I'm not calling endSessionAtSourceTime, just finishWriting.