Hi Liv,
It will be difficult for us to help you with this issue without being able to test your file locally and reproduce the error. Please refer to the forum guidelines and post your file 
From the get go I can suspect 1 thing: raytracing has a probabilistic sampling element to it I think and therefore the variations you see could be very slight variations due to that. Can’t confirm this without seeing your file or having a scale beside your pretty mesh though