Maybe someone can think of a reason that it should work this way
From a HW point of view, I can imagine circuitry that detects falling and rising edges, as well as levels, that would be active all the time (whether or not interrupts are enabled.) Enabling the pin change interrupt causes an immediate interrupt, because sure enough there HAS BEEN a falling edge some time in the past, even though it wasn't set up to cause interrupts. I think the timer interrupts work like this too. If you've been running the timer for a while without paying attention to the pieces that cause interrupts, the "overflow" bit will be set (you never cleared it, after all) and will cause an interrupt as soon as you enable timer overflow interrupts.