When you use symmetric/antisymmetric boundary conditions in a given direction, the data stored in the “rawdata” of a monitor is only for the points in the grid required according to the chosen symmetry.
For example, if you chose antisymmetric boundary conditions for the x direction in the simulation shown below, the grid used for the simulation only requires five grid points along the x direction; this is consistent with the dimensions reported in the field monitor’s rawdata. However, if you use the getdata script command to access the x coordinates of the field monitor you will find 9 values; the reason is that getdata automatically unfolds the data according to the symmetry/antisymmetry. You can still access the folded data using the option 1 for getdata, as shown in the screenshot below. For more information about the options in getdata take a look at: https://kb.lumerical.com/en/index.html?ref_scripts_getdata.html