Retirement tax questions

And regarding the withholding.  Apparently the withholding is on the net of SS-Medicare.

I looked in my SS file and the 15% withholding is on the net SS-Medicare.  Wonder why they do it that way?  Was checking for this post,

https://ttlc.intuit.com/community/retirement/discussion/withholding-on-social-security-benefit-when-...