Seems it was skipped due to the late running of the service. As it happens the LO service in question only made up 3 minutes of time. I've been on one which did the same, a couple of years back, running non-stop from Willesden Jn to Clapham Jn via the middle road at Kenny O and making up considerably more than that. I'm not 100% sure how common it is, but I've also been on a Southern service (just the once) where the same type of overtaking manoeuvre occurred, just like yourself, so I'd say it's by no means unheard of. Unfortunately removing the stops dramatically reduces the usefulness of the service, so it only seems to be done out of desperation to get the next workings on time, presumably once the delay exceeds around 12mins or so in order to stick to turnarounds at a minimum of every quarter of an hour for each route.
