Ryan
Actually EIGRP has 3 ADs to remember -
5 - EIGRP summary route
90 EIGRP internal
170 EIGRP external
as to why those number were chosen, well obviously they fall between ADs for other routing protocols but yes EIGRP external could have been made 190 if they had wanted. I'm not aware of any technical reason for this. Shame Russ White, a Cisco EIGRP expert, isn't on the forums much these days as i'm sure he would have been able to help out.
Jon