why an acquisition hash would be the same sometimes?

sorry for the noob question ,im trynna understand why an acquisition hash would be the same in some cases and why they would be different on the others. i tried modifying an evidence file and the acquisition hash stayed the same . i tried the same thing but with a different evidence file and the acquisition hash changed and its now different than the original file. either my encase is glitching up or i got myself confused lol

acquisition computerforensics hash