One reason I can think of is this - If you notice all control proesses (scope, time, cost), you make the update to the project management plan as part of the process itself. Note this can be in addition to the possible change made during the integrated change control process. But you may not really update the Project management plan as part of verify scope process instead you will perform the Project management plan update as result of the verify scope process in the integrated change control process. This is because you are working with the clients as part of Verify scope process and your focus is on validation rather than control. 

I can be wrong, I am just making a guess here. One more thing, I used to get a lot of such questions when I was studying for PMP, especially around OPA/EEF. OPA and EEF can apply for almost all processes, but PMBOK does not list them for few. You can argue over these, but I chose to just accept and move on, as it just doesn't matter - PMBOK is the BIBLE.