It doesn't use total curation rewards, it uses the 30 day curation rewards to calculate an APR based on your current HP.
So yes large swings of HP will definitely affect the APR shown, but it should normalize as days pass. (This is how it also works on Hivestats)
That both explains it aswell as fixes it! As in 30 days it would be back to normal. With powering down the difference is even smaller due to the weekly unstaking schedule.
Thank you!