Some muggleborns were coming from families, who supposedly had died out, but they didn’t, because of the squibs. Many adult muggleborns took their seat from Albus and were voting with the neutral section since they didn’t trust him with their seats due to the fact that he had kept them away from them. The school-aged muggleborns didn’t trust their seats and money to him too sadly and entrusted their seats to the other lords and ladies from the neutral fraction.