It often can depend on the length of each journey and type of road driven on, and not the overall annual mileage. A car doing 5k miles a year, but say once a week for a 100 mile round trip on fast-flowing A-roads, dual carriageways and/or motorways out of peak hours (so not in jig-jog traffic) will probably be fine, as the DPF will almost always passively regenerate or at least have more than enough time to actively do so during such journeys.

The opposite would be for a car that does lots of urban, slow-speed driving, such as a mummobile, even if that included many journeys and the occasional longer trip to friends and family but sparingly.

Of course, some diesel engines seem, howevere they are used, to be more susceptible to their DPFs clogging up than others. You may have been lucky if you were nearer to the first group if drivers, whereby a good long run at 50/60mph+ once a week would clear the DPF, or it could be pure luck that you did so less often but exactly when an active regen was required. I presume you've owned yours from new to give you a full ownership history to make the comments you did?