Overflows "O" messages with USRP X300

Hi all,
I try to use a USRP X300 with a 10G Ethernet link at full speed (200MS/s).
I followed all the tips from https://kb.ettus.com/USRP_Host_Performance_Tuning_Tips_and_Tricks
but I still receive "O"s from time to time.
I don't have these messages at 100MS/s.
I'm surprised to receive these overflow warnings, because this is not what is described in
https://files.ettus.com/manual/page_general.html#general_ounotes.
In this document, network-based devices are only supposed to drop ("D") packets because there is no back-pressure.
So how is it possible to receive "O"s (i.e. the device's internal buffers become full) with this setup?
Thanks for your answers.
Thomas