This is a curious little malady that makes you believe that you are much better at a task than you actually are but you lack the either cognitive power or self perception to realise how bad you are.
Consider of the HRF Hedge Fund Index versus the S&P500.
As you can see a simple index fund would outperform the majority of hedge funds in the world. Despite this glaring lack of ability there are literally tens of thousands of hedge fund managers who fail to see that they are rubbish. They go on collecting and losing clients money whilst making themselves wealthy. In average investors it has been shown that the the level of self perception among the worst performers is effectively zero. There is no reason to assume that this does not apply to investors of all sizes.