The main problem is that chess ratings are completely arbitrary. They don't measure anything absolute, so they can't be easily calibrated to match each other.
You don't seem to understand that ratings only compare you to other players in the particular group being rated, to help you find good matches..
Look at the graph of player ratings and you will see that your 1626 blitz rating puts you into the top 10% of blitz players here on chess.com.
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 1900. So, yes, maybe I am only an "A-player" at blitz, but that still does not explain at least 300-400 points of difference.
I mean OTB there is no chance on the planet I would EVER lose to a USCF 1550 at ANY time limit. They drop pieces and make basic positional errors. I mean if I put a clock in front of some 1500 at my club and played blitz it would be a joke, I would win 100 games in a row. In fact, even when I play blitz over the board with weak A-players (1800 or so) I crush them consistently.
So, what is the story with online blitz? 1600 players are actually candidate masters (2100)? That's what seems to be the case. The "pool" of online blitz players seems to be MUCH stronger than regular tournament players.