On interstate 270 in Maryland they have local lanes and the main lanes?

Every single morning, when I am headed southbound, the people exiting 370 to get on 270, they will always move over 2 or 3 lanes to the left to get on the main lanes. Why do they do this? They could cause an accident! Are they unaware that all the lanes go thru? That has to be the explanation cux there is no reason to cut over three lanes risking life and limb. Why are there no signs saying all lanes thru?