The CAP is guidance whereas the AIP is a published legal reference document. You should start from Annex 11 and PANS-ATM, go through the AIP GEN 1.7 filed differences and the GEN/ENR/AD text before plodding on to the CAP.
Perhaps the reason for A/G frequency calling is that several share the same frequency, and they want to know if you are calling them. Of course, if the relevant ranges were published... mmm... that might stop a well known Low Cost Carrier from calling from 150 miles away on a shared tower frequency to ask for the weather from FL xxx.