Laura Because I didn't have my own yet by that time. To see how it works.
Let my try a guess:
Your mom wanted to convince you to wear a cb. Then you tried her old one, which was possibly a little too wide. You found it not too restrictive and agreed to order one. Once delivered, your new cb was fitting and much more secure as expected.
Am I right?
At least for me, it was like that. When i agreed, i never expected it as that secure and escape-proof