Nothing "supposed" about it and you know that. 1.1 million apprehended in 2004, 40% of those in the Tucson BP sector. Over 400,000 illegal entrants turned back, a fraction of the total that cross.
Even that statistic is meaningless, since Border Patrol policy is just to feed them a meal, and drop them back off at the nearest border crossing, so that they can try again and again until they succeed.
Estimates of the "success rate" for one crossing are about 80 to 90%. So even if the Border Patrol sends you back, the cumulative success rate after 2 or 3 attempts is nearer to 99%.
That is, when being returned to Mexico, the jumpers don't go home to Jalisco or Chiapas. They just regroup and try again.
If the USBP apprehension rate is 20%, (a charitable guess) then multiplying that fraction a few times shows that in reality, the Border Patrol doesn't "stop" any crossers at all. They only delay and inconvenience them on their journey to America.