I imagine that the person that "fitted" this revolver barrel to the Luger ran up against the problem of thread angle (remember the preference for British "Whitworth" thread profile by DWM)...
Depending on the source of the "replacement" barrel, it's unlikely that it had the correct thread pitch and face angle, so any number of "adjustments" might have been committed.
This site describes the BSW thread:
https://www.boltscience.com/pages/screw4.htm
This is the Luger:
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Igitur si vis pacem, para bellum -
- Therefore if you want peace, prepare for war.