A number of times now, the cargo vehicles (Dragon, Cygnus, or HTV) seem to have arrived while the station was between crew switchovers.
With indirect crew handovers, three crew depart in a Soyuz, reducing ISS Crew size from 6 to 3, and a week or two later another Soyuz launches with a crew of 3.
What I have noticed is that a couple of times now, cargo missions have docked, while running a crew of 3 only. Since the cargo vehicles implies additional work. (They only stay docked about a month, on the US side, since with only one primary CBM port available (Harmony nadir. Harmony zenith is secondary) See Will the ISS need more docking ports for details.)
With only a month on station, there is a fair bit of cargo to unload, store somewhere (Tracked of course, else no one will be able to find anything), then refill the vehicle with downmass (Dragon only) or trash (Cygnus, HTV, Progress, ATV). There are requirements to balance the mass on reloading, so it is non-trivial.
You would think, targeting 6 person crew compliments would make more sense.
But is there a reason? Perhaps the schedule is so tight on the Visiting Vehicle Schedule they cannot 'waste' the two weeks when the crew is reduced?
Harmony, where the CBM for cargo docking resides, is really central to the station, (you must go through Harmony to get to Columbus (ESA module), Kibo (JAXA module) and rest of the station. So maybe a crew of three is easier, since there are fewer people in the way? Or moving cargo disturbs fewer people or experiments?