6965 SSI0 TNF status flag not accurate?

I'm working on a project that's reading the SD Card on the evaluation kit. I've taken example Fatfs code, and have rewritten the low level routines for performance (instead of pushing a byte into the SPI, and then waiting for the return, when I know I have to send or receive a number of bytes I make use of the hardware FIFOs to reduce the inter-byte delays).

I find that if I monitor the TNF bit in the status register to see when it's OK to push bytes into the FIFO, I don't always get back as many bytes as I have pushed into the FIFO. This suggests that I may be overrunning the transmit FIFO.

I've altered my code so that I just make sure that I've never pushed out more than 4 bytes than have read back (the FIFOs are supposed to be 8 levels deep) and don't bother with checking the status register before sending and the code works fine now, and is actually probably preferable for my purposes anyway.

But I wondered if anyone else had run into a similar issue and if there is some actual problem with the TNF status bit?