Acrobat cannot calculate Trim Box of oversized Illustrator PDF w marks
Fix your tool for the Illustrator to PDF feature you created.
It's calculating the margin difference at 1/10th so the math is wrong in presenting the true trim box size.
Mark A commented
If you generate the pdf without any bleed, the trim box is correct. With bleed the trim box is wrong. Unfortunately those of us in the print industry need bleed.