@lloyd1701
You're experiencing the Short-Fuse AP getting armed prior to entry of the Citadel.
Gibralter has a 5ms Fuse Time:
whereas Goliath has a standard 33ms Fuse Time (as well as a higher Arming Threshold):
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Gibraltar shells are wonderful against DDs because of priming off even very light armor and being able to explode quickly (instead of zooming out the other side of a thin ship before popping). This can make for consistent damage against Cruisers, but it can mean that spaced armor around the Citadel of a ship can remove the likelihood of scoring Citadels with Gibraltar AP.