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It is possible that the 310 BTC challenge is harder but has higher ROI. If the challenge is very complex, the guy might have just skipped the 0.3 BTC and focused on the big price.

Fits with the conspiracy theory that the 310BTC key wasn't actually in the picture, and the puzzle author just withdrew the money himself. This gives an excuse for not telling a solution, which would hide the fact that there is no solution.

Or fits the obvious assumption that 99% of people and effort were focused on the 310BTC challenge over the .3 one

If I'd focus on the 310, I'd probably do the .3 one first as 'training'. Then again, I hate puzzles so I am probably not representative.

One reward is a thousand times the size of the other - I bet that's the one people focused on.