In addition to ZFR's point about items like the girdle of piercing, certain armors give bonuses against particular types of weapons. At least that's what the handbook says, I never checked to see if it actually gets implemented. But this is maybe what you're seeing the in "y" values the computer gives to the monsters attacking your character.
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\"Though we name the things we know, we do not necessarily know the things we name.\" -Homer W. Smith
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