Quite likely the fact that so many illegal aliens are "visa overstayers" rather than "walkers across the Southern border."
Let's see.
According to most estimates, there are anywhere between 10 million and 20 million illegal aliens in the country.
From Xelor's reports posted here, approx. 500,000 of those entered legally, and then overstayed their visas.
This means that only 2.5% to 5% of the illegal aliens in the country, overstayed their visas. 95% or more, walked across the border illegally.
Now in this post, Xelor doesn't mention an exact number. But he's trying as hard as he can to sound like at least half the illegal aliens, are overstayers.
Another of the usual attempts to pretend illegal border-crossers aren't really the problem... when they clearly are. The overstayers are part of the problem too, but only a minor part by comparison.