Looks like
1 They have increased frequency.
2 They have introduced new promo fare.
To NRT 900yen. From NRT 1000 yen.
Regardless of the booking time
3 It's got more convenient for early morning departures.
1AM Tokyo --> 4:30 NRT T2 --> 6:00 NR T1
This is like a Japanese ski bus schedule.
http://www.keiseibus.co.jp/kousoku/day/nrt16_tm.html