They may be vain and prone to arrogance themselves. But they might not be able to see it as it can masquerade under other names.
For me, it’s always been cleverness. “wittiness”, “know-it-all”. These are forms of vanity/arrogance but I never see them as such. Yet when I see it in others, I call it “vain” and “arrogant”, even thought they’re doing the same thing I do… except I’m blind to it under those terms.
Probably for my own safety. After all, if I walked around knowing I have arrogant tendencies, how could I not then start changing that behavior, if I consider it a negative? So better to have it under other forms and say when it’s about me, “Oh I’m not arrogant… I’m [some substitute]” and genuinely believe it myself.
I barely have enough insight to see it in myself but I get glimpses of it at times.