Just curious, did you check to see if the SWG was off at the same time the bubbles disappeared?

As for the testing, some pool store employees could really care less about doing an accurate test so it wouldn't really surprise me that they get a different results. Some stores actually use test strips which are not very accurate at all although they have this fancy machine to read the color. TA should be one of the more repeatable test if they are using reagents and if they are then perhaps their reagent is bad, assuming of course yours is fresh.