This is expected behavior for the USB-7856R. For some FPGA targets, the requested host buffer depth for a DMA FIFO will not match the requested depth. This is documented in FIFO.Configure (Invoke Method) LabVIEW Help
. The actual size of the host buffer will depend on the target being used and the host operating system.
For the USB-7856R, the size of the buffer is dependent on the host operating system.