The speed reported by the bike will rarely match that of Zwifts, the bike does not know about in-game drafting, how different bikes and wheel sets effects speed, and virtual elevation changes.
How in-game speed is determined:
Watts reported, the weight entered, height entered, in-game bike used, in-game drafting (Tri and TT bikes get no drafting boost), in-game wheel set, in-game road surface, and in-game virtual elevation changes your in-game speed is determined.