Hello Michael,
Because of RadGridView's UI Virtualization model (it actually re-uses cells) you should reset the values of the properties you are setting when they don't meet your condition.
E.g.
You can read more about cell formatting
here and if you care to read about the UI Virtualization and how the logical structure compares to the visual structure, you can read more about that
here
If this helps, please remember to mark as answer. If you need further assistance, then please let me know
Richard