if you remove an element fomr the Resource Group the backup will be retained to give you the possibility to restore it according to the retention you specified.
I.e with retention policy of 2 backups, you have a RG with vm1, vm2, vm3
first backup will contain vm1, vm2, vm3
second backup will contain vm1, vm2, vm3
now you delete vm3
third backup will contain vm1, vm2
if the retention will delete the fist backup that means you will not have the possibility to chose 2 possible restore for svm3 hence you will see 3 backups.
this is a design choice, in the feature i think will be introduced the option to change this behavior..
hope that helps