If you’re using USPS, then my guess for the change might be related to whether Amazon considers Friday July 3 or Saturday July 4 (or both? or neither?) as the shipping holiday, combined with delivery expectations for your specific SKUs? 
If you’re using USPS, then my guess for the change might be related to whether Amazon considers Friday July 3 or Saturday July 4 (or both? or neither?) as the shipping holiday, combined with delivery expectations for your specific SKUs?