Ash I think AWOL's point is this...
If you change interval on the fly, at the time you change the LED state, the interval will be correct for the next pass with the LED in the new (on or off) state and will time that on or off section correctly. Then next time the LED state changes, the interval will get changed for that new (off or on) state. Difficult to explain in words.... yet another case where a flowchart will help.