So ,in this case we have pageScope bound variable "One"
No, "One" is not even in pageScope. It's a local variable.
${One + 1}
There is no scoped attribute called "One", so it's null. In an addition, null is converted to 0, so the result is ${0+1}, which prints 1.
${Two}
There's a scoped attribute called "Two" in request scope, whose value is "2". In an addition, a String will be coerced to a Long (calling Long#valueOf), so "2" becomes 2, resulting in ${2}, which prints 2.
${Two + 1}
Same as before. It becomes ${2+1}, which prints 3.