I have a group in ePO which is picking up machines from other group. I have the ip-range as well tag applied, however machines from other group are still showing up in this group.
* Make sure that the tag is applied in the Group details - Sorting Criteria(Select the tag which you have created)
* Switch to System tree then Action - Directory Management - Test Sort - By doing this only the matching systems will be in your group and remaining will be pointing to other group or Lost and Found.
Also let me know your IP range which you selecting, Because sometimes the OR \ AND might be the cause for this.