Answer
The number of sweep points can only be set to certain values.
The following steps are possible:
FSP: 125, 251, 501, 1001, 2001, 4001, 8001,
FSQ: 155, 313, 625, 1251, 1999, 2501, 5001, 10001, 20001, 30001
FSU(P): 155, 313, 625, 1251, 1999, 2501, 5001, 10001, 20001, 30001
If a number between these steps is set, the next higher value is selected. Coming back to the example:
If the number of sweep points is set to 1000, 1001 points will be selected.
In this case, set the number of sweep points to 1001 and the x and y-values will match.
The FSL and FSV(R) allow any number of sweep points between 101 to 32001 to be selected, so you will
not see this effect with these two instruments.