Replica and authentic jerseys are two different types of jerseys.
Authentic jerseys are the jerseys worn by professionals on the field (Vapor range for Nike), these are more expensive, and are designed for athletes (they have an adjusted cut). The notable difference with a replica jersey is also explained by the manufacturing materials used (in terms of lightness, ventilation and comfort). They are also distinguished by the fact that they are heat-bonded and sewn.
As for the replica jerseys, the jerseys are entry-level product. They correspond more to the jerseys of the fans (Stadium for Nike). The price of these jerseys is therefore lower and the cut of these jerseys is straight.
Replica jerseys are not counterfeit jerseys but entry-level jerseys.