Courtesy of Google Translate, I now understand exactly how the train <> timetable entry matching happens in arrival/departure mode. I feel a bit foolish, realizing that I'd basically been RTFMing wrong, but if anyone else is scratching their head over certain behaviors, here's a rundown of exactly how the mod works, and how to get trains to just pick the next departure.

=== How the mod works ===

The text in the mod's description is *exactly* correct:

--
Arrival/Departure: You can add arrival/departure timeslots. The train chooses the closest arrival time slot when entering the station, and then wait until the departure time.
--

This is true *even if the closest arrival time is in the future*. This means that if you ran this timetable:

[arr 1:00, dep 10:00]
[arr 11:00, dep 20:00]

a train arriving at 8:00, despite being solidly in the [arr 1:00, dep 10:00] timeslot, *will not depart until 20:00*, because the closest _arrival time_ is 11:00.

This was majorly messing with my terminal stations, where I just wanted trains to leave at the next departure time, always.


=== Making trains just pick the next departure time ===

Assuming a timetable with departures at 00:00, 10:00, 20:00, etc., we can make trains always pick the next departure time by doing the following:

[arr 00:01, dep 10:00]
[arr 10:00, dep 10:00]

[arr 10:01, dep 20:00]
[arr 20:00, dep 20:00]

...

The important thing is the addition of [arr 10:00, dep 10:00] timeslots; they make it so trains will never pick a future arrival that isn't the next departure.

Slack can be added by adjusting the first arrival time of the pair. For example:

[arr 08:00, dep 10:00]
[arr 10:00, dep 10:00]

[arr 18:00, dep 20:00]
[arr 20:00, dep 20:00]

Will let trains arriving up to 14:00 (halfway between 10:00 and 18:00) be handled as the "10:00 departure, running late."

Hope this is helpful for someone! This mod has completely changed the game for me, and it's been fun discovering how little I actually understood its behavior :)