Because clothes add permanent bonus once you aquire them

You mean, she keeps the power-ups from the old costume? If I wear "Yes, master" nothing will change? What if I wear another suit?

Here's how clothing works.

You get 1 clothing, and you get its buff into your power. The more clothing you get, depending on which buff it gives you it will all stack up with each other. Changing clothing doesn't matter since that would only change your look in the overworld and battle (sometimes only overworld since some older outfit doesn't have any in-battle sprites). You can view the stacked-up stats that you've gathered on the "Info" tab of the clothing.