Unclear hourly billing (double billing?) for T1-45 instance

Hello,

I'm using a T1-45 instance (GRA region) and hourly billing doesn't seem clear to me. According to what I found on OVH website, instance is billed for each hour. Ok, that's fine - but lately I unshelved my instance, used it for 1h 40 and then shelved it. Initially my bill increased by 15 PLN which seems fine if you assume it's not counting any taxes (7,5PLN netto per hour). But just a day later my bill magically increased by another 15 PLN even if in the meantime I wasn't using that instance at all. And to be clear, it's just server, not image snapshot.

Since it happened at least twice in the past - why I was charged twice and with delay?