Not always the case. Some older carbon steerers had a "thin" piece of AL tubing bonded into the top of the steerer. I have installed a very few of these maybe 10-15 years ago. Actually a really great solution until you want to transfer the fork to another bike with a significantly shorter head tube... If done well there's no "plug" slippage at all. Care does need to be taken as once bonded in place there's no removal, the couple I did had a lip that added a couple of MMs to the effective steerer length. Andy.