The pharaohs you might be thinking of could have been ones that came a long time after the pyramids. However, the more likely answer (in my opinion) is that the statement you mentioned isn't entirely accurate.

The (most reknowned) pyramids are dated from the old kingdom, but the chariot was introduced due to the "hyksos" invasion during the second intermediary period, after the end of the second era of unification. The Pharaohs buried with chariots are more likely those of the New Kingdom, an era during wich large chariot battles occured, like the battle of kadesh.