Yeah, to say that any system would use games not played yet in the SOS is just crazy, and one look at some of the still horrific SOS's of some B12 teams like Baylor at #305 would instantly disprove any such strange idea.
Ditto to everything Phil, Choke, and UcMiami have said, with the note that finding a fair and simple SOS rating system is tough because there are weighting factors involved. Playing a #100 and #300 team is pretty much the same for UConn, but it is much different for a #200 team. But to create an SOS system that fairly weights SOS for all teams is not easy. As UcMiami notes, weighting values for teams close to a team would be better, but there are still many factors involved with any range since the difference between a #16 team and a #35 team is huge compared to a #101 and #120, and there are always gradations involved. Also, some would say that allowances also have to be made for the fact that you can play a #5 team early on that loses its top two players and slides back to #50 by the end of the season.
I have no idea exactly how SOS is compiled for Sagarin because much of how Sagarin works is shrouded in a bit of "special recipe" secrecy, and anyone who tells you they know exactly how it all works is lying to you unless they are Jeff Sagarin and maybe Jim Sukup (not sure if he just compiles game results or works also behind the scene with the system). I am NOT saying that the Sagarin system merely adds up the opponents' ratings and divides by the number of games to compile the SOS and SOS rankings, as he actually makes some adjustments but it is fairly close to the mean average of the opponents. For St. Joseph's, ND, and UConn, the current numbers are as follows:
...Official...............................StJ................................................ND......................................................UConn
Sag SOS Rank:.....................6....................................................24.........................................................25
Sag SOS Score:..................79.51..............................................76.41.....................................................76.37
....Computed estimates
Sag Opp Rank Avg:.....88.14............................................109.23....................................................110.00
Sag Opp Score Avg:.....78.35.............................................76.74.....................................................77.88
I have no idea what and why Sagarin makes adjustments, but it appears that St. Joe's gets a bonus for having a higher percentage of sub 100 ranked teams while maybe UConn is penalized a little for having the most over 150 ranked teams. Hard to say.
Note that if the #150 team played the St. Joe, ND, and UConn schedules and everything had gone true to form by the ratings (lose to a #149 team and beat a #151 team), it would be 1-13 playing St. Joe's schedule, 3-10 playing ND's schedule, and 5-10 playing UConn's schedule. So everything is relative, and though UConn has by far the toughest schedule for a top 10 team to play, a weak team would actually do better with it than it would with St. Joe's.