Can be many reasons, I don't think anyone (at least not much) people/FFs can read AC's mind.
1. If your departure flight got delayed for 2 days, you can change your O/W SAN-YUL but probably have no say on changing YUL-SAN.
2. It can be US destinations are more competitive so AC wants to compete with other carriers. At least try to Price Match with other carriers
[KVS Availability Tool 3.3.4/Platinum - Sabre: Fares/TVN/CA]
Code:
SAN San Diego Metro / San Diego Lindberg Fld Int'l CA US = CLD SAN [KSAN]
YUL Montreal Pierre Elliott Trudeau QC CA [CYUL]
RT Mar-Feb
Carrier From To Fare Cur Expiry Min Max Fare Basis
--------- ------ ---- --------- ---- --------- ---- ---- ----------
DL SAN YUL 479 CAD 30 Jun T14WN90C
UA SAN YUL 479 CAD 30 Jun WA14N10
AA SAN YUL 479 CAD 30 Jun O14AQNR
NW SAN YUL 479 CAD 30 Jun K14APNR
US SAN YUL 479 CAD 30 Jun GXAUSJ6
AC SAN YUL 480 CAD 30 Jun A14STP
[KVS Availability Tool 3.3.4/Platinum - Sabre: Fares/TVN/CA]
Code:
SAN San Diego Metro / San Diego Lindberg Fld Int'l CA US = CLD SAN [KSAN]
YUL Montreal Pierre Elliott Trudeau QC CA [CYUL]
OW Mar-Feb
Carrier From To Fare Cur Expiry Min Max Fare Basis
--------- ------ ---- --------- ---- --------- ---- ---- ----------
DL SAN YUL 244 CAD 30 Jun T14WN90C
UA SAN YUL 244 CAD 30 Jun WA14N10
AA SAN YUL 244 CAD 30 Jun O14AQNR
NW SAN YUL 244 CAD 30 Jun K14APNR
US SAN YUL 244 CAD 30 Jun GXAUSJ6
AC SAN YUL 245 CAD 30 Jun A14STP
[KVS Availability Tool 3.3.4/Platinum - Sabre: Fares/TVN/CA]
Code:
YUL Montreal Pierre Elliott Trudeau QC CA [CYUL]
SAN San Diego Metro / San Diego Lindberg Fld Int'l CA US = CLD SAN [KSAN]
OW Mar-Feb
Carrier From To Fare Cur Expiry Min Max Fare Basis
--------- ------ ---- --------- ---- --------- ---- ---- ----------
AA YUL SAN 211 CAD 30 Jun O14AQNR
UA YUL SAN 211 CAD 30 Jun WA14N10
DL YUL SAN 211 CAD 30 Jun T14WN90C
AC YUL SAN 211 CAD 30 Jun A14STP
NW YUL SAN 212 CAD 30 Jun K14APNR
US YUL SAN 212 CAD 30 Jun GXAUSJ6