Lifekiller Adil

If the movie is supposed to be the prequel to the first installment of the series, then how did it show Optimus and the other autobots arriving in this very movie itself?.. Isn't it contradictory to the scene from the first Transformers movie when Optimus and the Autobots meet Sam Whitwicky when they arrive and Bumblebee was with Sam the whole time?...kinda confusing