There is no U fare bucket availability for those flights, only M and above which will likely mean much more expensive fares available as you have found.
ITA Matrix sometimes picks up incorrect fare bucket availability for certain carriers, showing fares that are not actually available to purchase.
Results from ExpertFlyer.com
Code:
Flight Availability Search
Departing JFK on 26/12/14 12:00 AM for CAI
Returning on 11/01/15 12:00 AM
Flying MS
Direct/Non-Stop Only
Exclude Codeshares
Frequency
Flight Stops Depart Arrive Aircraft Reliability Available Classes
0 Connections
MS 986 0 JFK CAI 773 Su,T,Th,F,Sa C9 D0 J0 Z0 Y9 B9 M2 H0 Q0 K0 V0 L0 E0 G0 S0 W0 T0 U0
26/12/14 6:30 PM 27/12/14 12:50 PM 72% / 25m
Results from ExpertFlyer.com
Code:
Flight Availability Search
Departing JFK on 26/12/14 12:00 AM for CAI
Returning on 11/01/15 12:00 AM
Flying MS
Direct/Non-Stop Only
Exclude Codeshares
Frequency
Flight Stops Depart Arrive Aircraft Reliability Available Classes
0 Connections
MS 985 0 CAI JFK 773 Su,T,Th,F,Sa C9 D9 J2 Z0 Y9 B9 M9 H0 Q0 K0 V0 L0 E0 G0 S0 W0 T0 U0
11/01/15 10:25 AM 11/01/15 3:25 PM 48% / 35m