I found this which explains it pretty well:

https://bitcoin.stackexchange.com/questions/915/why-dont-the-timestamps-in-the-block-chain-always-increase#917

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I liked the explanation of a miner incrementing the time to re-use nonces already tried.

I'm going to keep an eye out for it now.

I found that interesting too. I didn't realize you could run out of nonces!