We are talking about B7 here. During that period the writer was king. Terry Nation created the series, he wrote the entire first season. It was script edited by Chris Boucher.
I recall reading something about Chris Boucher admitting and explaining why some characters(actors) got all the good lines. With so little time for shooting, they couldn't do a lot of reshoots. Some actors delivered their lines word for word. Others did not. Naturally, the writers began to skew the scripts more and more to the people who would give the line as written. I believe that Paul Jackson was one of the ones not happy with the amount and quality of his lines. Logic suggests (but does not dictate), that writer and script editor were less than happy with his precision.
(Nonetheless, Paul Jackson was very talented, especially if he knew the character was playing the double-game indicated in previous discussion. I've been in acting classes and some of the best people were not great memorizers.)