Javascript ternary operator result

I am making small utility to compile javascript block using C#. I am trying to understand ternary operator's execution flow. Now when I am running a javascript using Chrome or Firefox :

var k = 27;
var o = 78;
var a = k < 100 ? o+=2 > 11 ? "T" : "F" : o < 100 ? "J" : "P";
alert(a);

It should have give me result "T" or "F" if "o+=2" returns false. But instead of those it returns "78F". Can anyone please explain me whats the logic behind it.