Why turret letters were never completely standard

How did naval turret lettering work, especially the use of Q, X, and Y?

There was a broad A-forward and X/Y-aft convention, but every navy could vary it and some changed between letters, numbers, and names.

The familiar sequence generally letters forward positions from A onward and uses X and Y for after turrets. This was a convention rather than a universal law: some navies used Z, and Russian practice moved among letters, numbers, and names. Even one navy could change its policy over time. A supposedly universal turret alphabet is therefore a poor basis for inferring a ship’s exact layout.