A poll doesn't include all members (only a sampling), and favorability isn't a net sum between all candidates. Just because Hillary had a 75% favorability doesn't mean Bernie had 25%. He could easily have had a comparable favorability (we don't know because the poll wasn't released). Union members tend to have favorable views of Democrats in general, so Hillary having a high favorability isn't surprising. Without seeing how it compares to the other candidates it's not very telling.


Also the poll that mentions the 75% was taken no later than September (according to this politico story). The endorsement came two months later in November. There's no telling how the polls could have changed in that time since Bernie made substantial ground in name recognition and general polling over that period (that's assuming their leadership even bothered re-polling before they made the endorsement).