Why even an aggressor may start an unready war

Is an aggressor at least more likely to be ready when war begins?

Not necessarily. Nazi Germany in 1939 shows that the state starting a crisis may still misjudge when and how a larger war will begin.

Strategic initiative and military readiness are different conditions. An aggressor can choose an action while miscalculating the response it will provoke. Hitler did not know that a general war would begin in 1939 on the resulting terms. Armed forces often enter conflicts before their planned programmes and preparations are complete.