The bundle has a progress bar whose texture is 64 pixels wide (not counting the border), allowing for a perfect translation into the used space, as the total weight is also 64.

However, instead of increasing 1 pixel per item, it increases 2 pixels at the start and 2 pixels at the end, making all the progress in between slightly squished, which often translates to inconsistent half-pixels on screen.

This can be observed better with GUI scale 0 and a resource pack.