How to adjust DRX_TUNE1b correctly?

I have troubles to settle correctly DRX_TUNE1b. Indeed, for 110kbps bitrate, DW1000 manual recommend to put it with 0x0064 for >1024 preamble length.
However, dwt_configure in Decawave API set this value like this :

For all preamble we set the 0x0064 value. So why API doesn’t correspond exactly to manual ?
Thank you for reply.