The school doesn't know who put them first/second etd
The get the list of everyone who has applied, (but not what preference that was for them) and then rank everyone on that list according to their admissions criteria (or the LA do this for them, it depends on the school)
The LA then take the ranked lists from all the schools & do the "sums" to work out who gets offered which school, giving everyone their highest preference school where they got a place (unless they didn't get a place at any preferred school of course).
eg
Assuming 3 children put schools A,B&C as their choices, in ranked order
Child 1 - ABC
Child 2 - BAC
Child 3 - CBA
Child 1 could in theory a place at school A, B & C. As school A is first preference they get offered that & "free up" the places at schools B & C for another child.
Child 2 didn't get a place at B, but did a A&C, so is offered A
Child 3 didn't get at C or B, but did at A, so is offered A.
For school A Child 3 could have actually been higher up the ranked list (by admissions criteria) than child 1 & 2, and child 2 higher up than child 1, you can't tell this from which preference number school they got the place at KYSWIM (it's diffucult to explain!
) So I can't see what point the school are making by saying "everyone who put them first got in" 
It's an "equal preference" scheme, NOT "first preference first".