Because he might have been mining with a node that was not connected to the rest of the network.

If you mine a block that is not accepted but you keep mining the next block you find will then cause the 1st block to show 1 confirmation if you keep mining that number will keep growing. But you are mining on a fork that has nothing to do with the rest of the network.

Not quite the same but: https://bitcoin.org/en/alert/2015-07-04-spv-mining

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Either way it does not seem that those are real mined coins that are in that wallet.dat

-Dave