"Stranger than usual" could be an evidence for not being the real Dumbledore, but instead Voldemort's imaginary Dumber-dore. We were already shown in the chapter that Voldemort had a wrong model of him.

(Maybe acting like insane was Dumbledore's long-term strategy to prevent his enemies from simulating him precisely. But that seems rather paranoid. On the other hand, he sometimes works with Moody.)