I just posted Setting SPI clock frequency on Task Tracker. Feel free to discuss and make suggestions here.
The SPI clock goes through a divider coming from the system clock. So, for example, if the source clock is 100mhz, you can get 50mhz out but 90mhz would be impossible. So, when you set the clock to 90mhz, you end up with largest possible value that matches or less than what toy set it to. Inn this example, you set it to 90 but you get 50 and that is the expected and proper behavior.
Does this cover the question?
If I look at the table, it seems to say that the “largest possible value that matches or less than” isn’t holding true. If you set 1khz, and end up with 330khz clock, that’s not “less than” by a healthy margin. Perhaps the clock dividing isn’t taking effect ?
@ Brett - I just noticed those. There maybe an error at lower clocks since we never test them. I have never seen an application with spi running in the khz range 
Setting SPI clock frequency was updated.
Status went from Proposed to Closed.
Priority went from Unassigned to Trivial.
[line]The behavior observed is correct. The lowest possible SPI clock on the G30 is ~329 kHz, ~352 kHz on the G80, and ~510 kHz on the G400. This will be documented in the next datasheet update.