dear Jack!
I am thinking that it is because when network does not provide A5/1 ciphering algorithm, then it is mandatory to insert RR_Classmark_enquiry transaction in Location_Updating procedure (In MM_LU_request there is Classmark_1 information element which contains only A5/1 support indication). Hence SDCCH load is increased.
When A5/1 ciphering algorithm is provided in your network then it is not needs for Classmark_2 information element which contains A5/2 support indication also.