Is this because we are declaring the variable inside of the callback function that the value of statusHTML gets (resetted?) ?

Yes, that is more or less the right answer.

A variable declared with var has a local scope, also called a function-scope. That means that it only exists within the function it is declared in. Since the two XMLHttpRequests gets assigned different functions you can declare variables with the same name without creating a conflict. So even though they share the same name, the statusHTML in the first callback function is not considered to be the same variable as the one declared in the second callback function.