E.P.
My DD was also a spotty sleeper when she was an infant and now, at almost 12 months, she has two chunks of time when she takes naps... but I never know exactly how long those naps will be. Sometimes the afternoon nap is 40 minutes, sometimes 2 and a half. Sometimes, when she wakes up after a 1 hour, she decides she's still too tired and falls back to sleep. I think she needs more than just 1 hour, but if she's not going to sleep then I've found it's just not worth fighting or fretting over it. It's better to just move on and keep her happy (hence, also keep me happy).
I think babies in general have completely different napping tendencies, and our girls seem to be similar in that they're unreliable. I don't know why they're unreliable, perhaps they just haven't developed the means/desire to sleep for a set amount of time yet, but I do know that it's not really worth dissecting too much. She may develop a more reliable schedule in a few months, but she may not. The important thing is that your daughter is still generally happy and healthy.