But we don't read linearly. E.g., as I read “The average speaking speed is around 150 words per minute” my eyes flicked back to the equivalent sentence in the previous paragraph to check the comparison of the numbers. Listening to somebody drone on live you can't do that. Even watching videos for entertainment I sometimes jump back a bit to make sure I got something but it's still much more hassle than just moving your eyes up for a second or so. Consequently, a decent lecturer will repeat things in some form (e.g., inserting “so 2/3rds of the reading speed” after “150 words per minute”) in a way they wouldn't do in writing. The word rates just aren't comparable.