I think it's a Port Authority thing, as there's a 10-trip discount for JFK AirTrain, too.

I think both AirTrains are $8.25 for a single-ride (at EWR you'd only pay that if you had an NJT or Amtrak ticket to a destination other than EWR and got off at EWR--for example, if you had a monthly NJT pass or if you wanted to save 25 cents by buying a ticket from NYP to Elizabeth and getting off at EWR). But the JFK 10-trip discount is much better: a 10-trip JFK AirTrain metrocard is still only $25 (I think they keep forgetting to raise it), representing a $61.50 discount, while the 10-trip EWR NJT ticket is only a $37 discount.