It's all probabalistic; you could in theory mine a block by hand if you got absurdly lucky. Think of a machine with 8 GPUs being like dropping a million balls into the bingo jar, and coinhive like dropping 1 or two balls in for each visitor. If you get enough visitors, some of your balls will get drawn occasionally, making you money.
The reason GPUs and ASICs are used isn't because they have a better chance per se, but because they are more power efficient. With coinhive, the person who benefits is not the person who pays for the power, so that's not at issue.
It's repeated over and over that GPUs and ASICs give you a pretty big advantage over CPUs, but that's not true at all for cryptocoins such as Monero that use CryptoNight, that is why it's one of the only coins that makes sense to mine with a botnet or JavaScript.
The reason GPUs and ASICs are used isn't because they have a better chance per se, but because they are more power efficient. With coinhive, the person who benefits is not the person who pays for the power, so that's not at issue.