That's completely normal, some are fixed and don't spin and some are locked on with a little retaining pin and they spin.
You should be able to look carefully and see the little pin that rides in the groove. Long as it don't pull off, nothing to worry about. I think your probably ok.
In Memory of Paul "Dietrich" Stines.
Dad: Say something nice to your cousin Shirley
Dietrich: For a fat girl you sure don't sweat much.
Cue sound of Head slap.
RIP Muggsy & TMan
"If you are a warrior legally authorized to carry a weapon and you step outside without that weapon, then you become a sheep, pretending that JOCKO will not come today."