PXI-6624 encoder count limits

Hi,

I'm using a PXI-6624 card to read from a quad encoder with 1024ppr using Veristand configured to read in X4 mode. 

At approximately 11710RPM my signal saturates and doesn't increase anymore. For the 1024ppr encoder, at the saturation speed of the 11710RPM im seeing, the encoder signal is 200kHz for each encoder line (A,B) and pulse width is approx. 5usec

The PXI card states that it can read up to 400kHz with a signal min. pulse width of 1us. 

Can someone clarify what I am missing? 

Regards,
Dave