I think it might be because good developers filter you out before you filter them out. They might spend time working on the code because they happen to be inter

I think it might be because good developers filter you out before you filter them out. They might spend time working on the code because they happen to be interested in the problem, but a good developer would not waste their time on dull problems just to get a phone interview because they don't need to. Especially if they have never heard of your company. If you are selecting people who are desperate for jobs, your P is not surprising. Perhaps your should fix your own (hiring process) bugs first? Maybe P(great developers applying)=~ 0.00016? Just kidding :D