SeaSonic are truthful in what they are, and provide a quality unit. I believe they have 620W and other 'odd' numbers.
They make plenty PSUs that are 50W or 100W incriments.
Some lower quality PSU manufacturers (or "branders") outright lie about the max output or capabilities of their PSUs, rounding them up to a 50 or 100W incriment. (and sometimes, I believe, just pull a number out of their....). That really distorts the market "standard" of 50 or 100's.
IMHO, if every PSU was rated accurately, the market would be full of random numbers. 17's, 4's etc. Then you could spot the truly quality units a mile away.