Steps to reproduce:

1. Open attached project
2. Open scene "test"
3. In hierarchy, expand "Canvas" and select "Scroll View"
4. Run scene and notice a number of callbacks printed, should be 78
5. Exit play mode and change Scroll Rect component of Scroll View. For example, add empty OnValueChanged event by clicking "+"
6. Run scene again and notice that this time number of callbacks is much lower - 40
7. Exit play mode
8. Enter play mode again
9. Notice that this time number of callbacks is back at 78 again

Note: happens doing other changes too: changing movement type, enabling inertia, etc... Editing Image component doesn't mess with number of callbacks

Reproduced with: 5.2.4f1, 5.3.4f1, 5.4.0b10