The rate of development is the pace that a child develops at, this can be the pace within each sequence or the pace over all and goes to cover all the set areas or periods in between or altogether in the sequences.
These principals run through all areas of development from mental to physical to emotional, no matter what the age of the child. If one is skipped or is slow it can be a cause for concern and may lead to a child being given a special recommendation or having a special need in or outside school. Growth and development are a continuous process and are different for all individuals.
The rates all move from the general to the specific, for example from moving a hand in any direction to grasping a pencil or catching a ball.
So the variation is to do with the building blocks of development, which is sequential development versus the rate or the speed of development which is the rate of development. Both are obviously interlinked.