I needed time to understand the question, and here is my answer now. I do hope it fits your question though 😀
the paging SR at core network side is different than the one at radio side because they are not counting the same thing.
The CN counts whether a paging sent has received a positive paging response from a MS.
The BSS counts wheter a paging received from CN was succesfully tranmsmitted over the radio interface. There are several possible cause from failure: BSC congestion, Abis congestion, BTS congestion, PCH congestion. The latter being the most common.
Bss doesn’t know if one paging message got a paging response or not.
CN only cares about that.