This would be a phone specific question.
The difference between a normal paging and the multicast paging is that the normal paging addresses each individual device first, then gives the signal to the person to page their message.
Multicast on the other hand, the phones are actively listening to the multicast address and when the signal comes they open up straight away and page the message.
Essentially, with multicast "Aint nobody got time for the beep"