Quote Originally Posted by Curmudgeon View Post
"Different" is your own addition, something not specified in the rules. As already stated, absent such special language as "different" or "random", the singular article indicates just one set of normal clothing as per mattie_p's answer.
Being the same is not stated in the rules either.

Quote Originally Posted by Curmudgeon View Post
Beyond not having any language which would allow more than a single set of clothing, the armor property of course lacks any mechanism whereby the user could make a selection if more than one set of clothing were possible. Positing a difference in clothing from one activation to another is not backed up by the rules. Magic in D&D performs as written, and never gains capabilities beyond those specified in the RAW.
The magic item also lacks a mechanism to determine the look of the single set of clothing nor does it specify that this appearance is chosen at the time of enchanting the armor. So the item would either not work or would change into a "set of clothing" that has no properties such as shape, color, texture etc. Which defies the description of normal set of clothing.