current_parameters in host refreshes all host parameters including those that are inherited from hg, os, etc.
In hostgroup the only parameters present are the hostgroup's and it's ancestor's group parameters.
When the parent changes the parameters are already refreshed through process_hostgroup.
current_parameters is invoked in other cases (like changing an os) that are relavant for host but not for hostgroup as they have no affect on hostgroup's parameters tab.