VisIt tries to minimize the amount of work that it must do to generate a plot so that it can be done quickly. Sometimes databases have variable information for each material in a cell instead of just having a single value for each cell or node. Because the variable is defined for each material in the cell, these variables are known as mixed variables. VisIt tends to just plot the value for the entire cell since it is more work to go through the material interface reconstruction (MIR) stage, which is usually only done when removing material subsets but is required to plot mixed variables correctly. You can force VisIt to always do MIR by checking the Force interface reconstruction check box. This will make mixed variables plot correctly even when you are not removing any material subsets.