It depends on what time of the day you test. During your photoperiod (when the lights are on) your PH will rise and it'll fall after the lights go out.
If you're testing at roughly the same time of day and getting PH swings like that you should look into it a little more though.
That is normal, and that's fine. You can minimize pH swings by adding a refugium to your system and running the fuge lights opposite of the display lights.