Thank you for clarifying.
I have checked the case in details and I must say that this would be the expected behaviour. The logic is that this (the selected page number) is the 10th after the first in the list of shown page numbers. For example:
1. You have 1-9 (Selected 1)
2. You have 6-13 (Selected 10)
3. You have 11-18 (Selected 15)
You can check that the MS DataPager does the same.
All the best,
the Telerik team