Thancred is an odd case, and it comes down to the change in development team back in 1.0. Originally it seemed Tanaka had planned on eventually introducing BRD to the game in a completely different way to how Yoshi-P eventually did - it seems it was meant to have been just a class and not a Job, hence why Thancred uses throwing knives but still calls himself a 'bard'.

Naturally when Yoshida became Producer and introduced the Job system, he had BRD come off ARC and use the ARC's weapon of bows, making Thancred an orphan class-wise. So in ARR he was retconned to use GLA abilities while still being loosely called a 'BRD' (even though Armoury System wise he most definitely is not). I guess this is also the reason for his thief/rogue-type backstory in the 2.2 storyline - it's to try and explain the retconning to his abilities that have been made.