It can't happen. Neutrality in aqueous solution means equal quantities H+ and OH-; since your solution is unlikely to have a net electric charge, so too must your anion and cation quantities be equal. Buffering from acetic acid being a weak acid is meaningless if you don't have other anions around to make up that balance.
Solution is electrically neutral:
[H+] + [Na+] = [OH-] + [AcO-]
Solution is pH neutral:
[H+] = [OH-]
Substitute the latter in the former, subtract, you can only get:
[Na+] = [AcO-]
If the concentrations of both solutions were the same, and you're titrating by adding both to the same volume, you can only have equal concentrations if you add equal volumes.
If this was not the case (as you describe) we have to violate one of our assumptions:
- Solution actually had a net electric charge (you're doing the titration in space and you're bombarded by beta radiation, i.e. unlikely)
- Solution was not truly neutral. Maybe someone calibrated your pH meters wrong, maybe everyone titrated with a faucet while texting and over-shot by a factor of 3.
- Contamination with a strong acid added a variable we don't know about to the ion balance.
- Most likely, the solutions were not truly the same concentration. See if your teacher made her NaOH solution from solid NaOH that she let sit around in open air. It can pick up a lot of moisture from the air, i.e. it's hygroscopic, and before you know it the weight you're measuring on a balance is half water.
