The captchas are not case-sensitive, in all I've tested there I used lower-case only as well as upper-case only and case-sensitive solving...
In all 3 cases the Accuracy is there, very low...

So, I still have winnings (tested only the default blue-captcha which it's always upper-lower case) using lower-case only ...

The conclusion is that you are not actually get paid for what you resolve correctly but for around half of it, and you are misled by the delayed update of balance.

I assume that the website owner is increasing his profit by reselling the captchas, giving away half of what he actually promised, 5 uBTC.