Total wagering volume.
According to the following post made by TheQuin, you can also deposit 100 dollar and get rid of the captcha without any need to have big wagering volume.
Thanks.
Finally got some answer about this mystery XD. Don't really know about these, I tough only an old account who have the ability to claim a faucet without a captcha.
I think that still applies. It's been a long, long time since I had to use captcha but IIRC correctly, there was a certain amount you had to wager OR deposit before the captcha applies. I currently don't have a balance at $100 but still works for me, but I'm very close to max freeroll bonus BTC (and that comes from wagering) so I guess I hit the wager requirement. Very sure the explanation is somewhere, just can't be bothered to find it as I don't need it:)