In reply to:

---

But wouldn't it work just fine if the header said 'the track is a tenth of a second (or, say, 1 frame) longer than it actually is' ?

---

I wouldn't expect so - but then, I haven't actually tried it. Have you?

I think the track-length thing is a red herring, and that it's the presence of the VBR header that's causing the gap to appear. Some tracks without a VBR header appear shorter than they actually are (if they begin with a high-bitrate frame). Pink Floyd's "The Wall" tends to encode as a mix of longer and shorter than actual.