Halo Timeline Confusion

In Halo 3 there were many new vehicles and weapons (because it is in the future), but in Halo Reach its in the past which is why it is understandable that many of the vehicles and weapons from Halo 3 arent in there, but why are there more powerful weapons and such if its in the past and they aren't in Halo 3? I have been thinking about this for a long time and wanted to know what everyone else thought about it.