Anything in double-quotes is a string literal, meaning it will return exactly those characters.
So "AJAX.LastData" will return some text which just happens to look like an expression. Without the double quotes, AJAX.LastData will retrieve the last data from the AJAX object.
I guess you're making the same mistake accessing the stat1 variable. "stat1" will return some text that says "stat1". Without double quotes stat1 will return the contents of the global variable stat1.