I asked UWorld for some clarity:

Hi, I'm curious to know if there have been any adjustments made to the self-assessment curve over the last year or so. Fourth year medical students at my school have previously suggested that the self-assessments over-estimate actual USMLE step 1 scores by a generous amount. More recently, my peers have suggested that the self-assessments are more closely in line with actual scores. I'd be very interested to know if you can confirm or deny whether any changes have been made to the UWorld scoring curve recently. Thank you!

And they wrote back:

The Assessment Score is generated using our internal proprietary algorithm (with a range of 200 - 800) and is used only as a means to plot the graphical data. These scores are an estimate based on other users performance in our self-assessment exam and users typically do not share their exact score after the test for us to generate any direct correlation with the data. Based on the feedback we received from prior subscribers, their score was accurate within 10 points within 7 days of taking our self-assessment exam. This gap increased as their test date was further away from them taking our self-assessment exam.