It's a speech impediment; he has trouble saying R's (and has improved as he's narrated more and more videos).
Isaac's earlier videos would show Elmer Fudd with the message "Having problems understanding me? Turn on CC." Example: https://youtu.be/gOu3zGfP-TQ?t=40
Source: I've watched lots of his videos and highly recommend them—mind expanding ideas!
Thanks a lot for the context. I have to say I didn't really have a hard time understanding him. I didn't even consider the possibility that it was a speech impediment.
Isaac's earlier videos would show Elmer Fudd with the message "Having problems understanding me? Turn on CC." Example: https://youtu.be/gOu3zGfP-TQ?t=40
Source: I've watched lots of his videos and highly recommend them—mind expanding ideas!