I scripted 600-900 in 1,5,10,25,50,100 increments. My units do 600 or 50m increments.
It's because both cgminer and cpuminer set frequencies based on predefined init messages, and they've only made a few of those. These are the frequencies supported by cpuminer:
static const char *str_frequency[] = {
"250", "400", "450", "500", "550", "600",
"650", "675", "700", "750", "800", "850", "900",
NULL
};