Fiddle:
https://fiddle.sencha.com/#view/editor&fiddle/304t


Click button "Get CheckBoxGroup's Value Only", the value of the checkboxgroup is { "cg": [1, 2] }
Click button "Get Values", the values of the form is { "cg": [{ "cg": [1, 2] }, 1, 2], "rg1": 1,"rg2": 3 }

Classic CheckboxGroup doesn't have this issue.