You are not receiving less, you are spending more on additional insurance that itself went up in price. If you cancel that extra Rx policy (not that I recommend that!) you will see. Medicare will not tell you "why" - because that is a legislative thing - Congress puts these rules in place.
So, to repeat, there is no law that says if you spend more on better insurance or to keep the same extra insurances you choose to buy, you won't pay for it, you won't find that. It doesn't exist.
Again, you are confusing the "hold harmless" law, the fact that they can't charge you for more of the Medicare B premium increase than than your COLA can absorb, so Part B increase can not be the cause of a lowering of SS. However, other expenses of yours, including Medicare Supplemental policies, can. There are other things that can as well - including child support, alimony, valid garnishments, etc. But NOT the Part B premium increase.
I would add that your question was: why am I getting less per month in 2017 than I received in 2016?
That has been answered: Because you are opting for a drug plan that increased in price by $10.20.
You indicate you would like more background. Here is your start:
Here is more: http://www.nolo.com/legal-encyclopedia/hold-harmless-rule-protects-social-security-from-medicare-premium-increases.html
Hope this helps? It can be worthwhile to shop around for drug plans, if you think yours is getting out of hand.