At the assembly/bytecode level, you still have to save the return value of the recursive call and then do something to it, whereas a tail call wouldn't need to save any intermediate value because it would immediately return it as soon as it got it.

I guess Java's infix notation kind of obscures this. This is one of the reasons why functional programmers prefer to consider everything a function. It's also why some languages (like Clojure) have special syntax for tail calls, with special error messages if you goof up the semantics.