I have noticed this only rarely, so it could be related to what you eat the day before I suppose... If your diet primarily is clean, balanced and raw, I suppose the results would be less dramatic.
I have noted however, having used both a refined sunflower and unrefined sesame (I have not been able to find an organic cold pressed, unrefined sunflower in a glass bottle) that the unrefined sesame oil is stronger in taste AND heavier, which I suppose is to be expected, making for a heavier pull.
Where I may have to do 2 10 minute pulls with sunflower, I never had to do this with the sesame.
As my wife prefers the Sunflower oil, we went back to it, I would like her to keep at it as she has struggled with very bad teeth and gums over the years.