I heard several explanations.
1. They put in a little oil each day and it still lasted even though it was an 8th of what they really needed
2. they didn’t even have enough for one day and the little they did not only lasted 1 day but also another 7.
3. Beating the yevonim is commemorated by the first day and the other 7 are in addition to that day (I don’t really understand this one)