YA/CA is the "PerkPass" option for NW FFP members (applicable only to NW/KL/CO flights), and it should correspond to the "open miles option" for FB.
For flights, which show YA-/CA-, have you tried asking FB if the "open miles option" was available (and vice-versa)?
I tried AMS-YYZ
15/02 CS+CA+ awards available/open miles available
16/02 CS-CA+ awards not available/open miles available
18/02 CS-CA- awards not available/open miles not available BUT there are "normal" seats available.
So YA/CA is indeed for open miles, and I learned also that open miles are not always available when paid seats are...