Or you could have units that never get past 420 GH/s.
I had the same thing until I added more power or used better PSUs, even going from using a piss-poor 850. If you have other PUSs handy, it might be worth it to test that and rule it out to be sure. Other things could be network problems, pool problems, using the same miner, sub-optimal miner difficulty settings, or even the already-noted S3 mining rate instability. I'm sure you've already ruled out most of those, just throwing them out there anyway...
That's not what I've seen from testing my S3s. I set up and configure every single one of my S3s on our testbench before they get put into production, and every single one is different. We use the same PSU (Rosewill Capstone 750-M), same killawatt, same laptop to set them up, same switch, same internet, etc.
Most of them are running at 238, so that's my basis for all my numbers. Sometimes there are 15W swings in the power consumption, and up to 20GH/s swings in what each Ant settles down for an average speed after 12 hours. Remember, this is all at 238Mhz. Obv moving up to 250 or down to 225 affect the power/speed, but I'm just comparing all my units at the same clock speed, and they're all different.