Why is my blitz rating on 600 points lower than my USCF

I don't understand how it is possible to be a 2100+ USCF rated player and have a rating in five-minute chess of 1550. Some of the difference might just be that I am bad at thinking fast, but, come on, 600 points??? Judging from my opponents' opening book knowledge I would put them as mid A-players, about

Average Player Rating? It Depends.

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