Question about the "sigma" number in the likelihood scan

Dear experts,

I have a naive question w.r.t to the plot attached.
As you can see, it is a likelihood scan plot with several dashed line indicating 1 sigma, 2 sigma and so on. My question is that why the 1 sigma line is at 1, 2 sigma line is at 2^2 =4, and 3 sigma line is at 9? I suppose it shouldn't all be the integral for the 68-95-99.7 probability.