You should have done a search in the "RC Radios-Receivers-Servos" forum. I did it for you. This is "Danny from JR"s answer to your question (it was asked before).

That is just a choice that they make. They have selectivity and sensitivity listed, and they have selectivity listed as a db value.

I do have a question for you. I have been in the RF industry for 14 years. I see people asking this question all the time, but I never knew why they bother (You cannot change it, modify it, or do anything about it). So, why do you want to know????