I had it once and it was spot on....Low battery.....:P
I took it to Volvo for a cambelt change and mentioned it, I was told they were amazed it even started it was that flat. I'd get it swapped out ASAP.
Remember, an OK voltage doesn't represent current (charge) inside the battery so a multimeter won't always help unless it's very flat. Get it on a proper battery loading system to check if you're worried about it.
PS, from previous threads I've read, if it was your alternator you'd have christmas tree lights on your dash and potentially, depending how it's failed, smoke coming through the cabin