Look at the numbers, and something of a mystery emerges regarding the Trump administration’s claims of a surge of uncontrolled arrivals.

Net migration across the southern border is at historic lows. Border apprehensions of unauthorized arrivals were at the lowest in modern record in 2017 and 2018. Apprehensions are up this spring, but still far below the highs of the 1980s, 1990s or early 2000s.

Yet processing centers and detention centers are filling, some to near-capacity, straining to meet even basic medical needs of migrant families. Images from the border show women and children huddling in overcrowded facilities.

The answer, immigration experts say, is that the crisis isn’t primarily caused by the number of arrivals, but by Trump administration policies that disperse them to a handful of centers and that treat families who voluntarily surrender as invaders — creating the appearance of an invasion.