As far as Bill W and the Big Book it is typically agreed that Bill was indeed a wordsmith and used many synonyms to express thoughts, not wishing to be redundant. His use of the word shortcomings and defects are one such example, although I've have been told meeting topics have surrounded these words.
"Thanks for your inquiry, requesting to know the difference between ‘defects of character’ and ‘shortcomings’ — as those words appear in the Steps. Actually I don’t remember any particular significance in these phrases. In my mind, the meaning is identical; I guess I just used two ways of expression, rather than to repeat myself. It’s just as simple as that."
According to G.S.O.’s archives Bill W. commented about his use of “Defects of Character” and “Shortcomings” interchangeably in the Steps in a personal letter he wrote dated March 7, 1963.
It seems likely the same is the case for the use of both words - slip and relapse in the book Alcoholics Anonymous. Simply synonyms.
Perhaps much like shortcoming/defects we choose to differentiate the two words.