why PXI5124 have missing code?

[learner xu]

I did a linear test to PXI5124 with PXI5422 as the FGEN .I got a histogram  like the picture down.I don't know why.I made a programm using labview and I set the range of the scope 2V Vpp,then the supply is 2.5V Vpp. I wonger why.