Because it has shown up in a few bug reports:

Ambush skill is like any other skill, in that it has a delay betwen uses.
However, unlike most other skills, the delay is _after_ the skill is used.
That means that monsters with low ambush percent can try to ambush everyone
they see, but will usually fail to do so. Example: if the first player (let's
call him Lionbait) enters the lair of an aggro monster, the monster will
almost certainly attack and try to ambush him. Maybe he succeeds, maybe not.
If a second player (let's call him Luckponk) enters immediately afterwards,
the monster might not be able to attack or ambush him, as it is too busy
killing poor Lionbait, even if the previous ambush attempt failed.
As such, Luckyponk can make a run for it free of charge -- how much time he
has is based on the monster's ambush percent. This is of particular interest
to parties, in which multiple players enter in rapid succession. The monster
has to pick one quickly, and often goes with the leader or a tank (something
that healers will be glad to hear). This is considered fair and in balance, is
tuned that way on purpose, and as such, is not a bug. But thanks for reporting
it.

Note that there is an exception. If you have attacked the monster, all bets
are off. To preserve fairness and balance, all monsters get a free
ambush/attack on any "repeat customers" with 0 ambush reload time. You can
only fool them once.

Shinarae Lluminus