On the essential difference between majority and permanent

Hello Rabbi!
I intend to ask a challenge question posed by the head of the yeshiva where I study matters of “majority” law and “fixed” law in Spikot.
In the Gemara in Ketuvot, the scripture states that when a person finds a piece of meat in the market when there are 9 kosher shops and one is a trap, the law is to follow the majority.
In the second case, when he bought a piece of meat in a kosher store and then settled where he bought it, the ruling is fixed and each fixed is a half-for-half payment.
And I was surprised by that. After all, from the perspective of logical intellectual explanation, it is most correct and safest to always go with a fixed one!
For example, when a person is brought a glass of water and told: There is a 10 percent chance that this glass is poisoned, but there is a 90 percent chance that this glass is fine and not poisoned.
The most sane and logical answer from the human mind’s perspective is to state that there is doubt here. Either it is poisoned or it is not. The same goes for meat in the market.
So then, it turns out that according to Sabra, the norm is to say “permanent” and not follow the majority !
So that’s why I asked to clarify, assuming that it is most correct from a theoretical perspective to go half and half, then, in what cases do I prefer the majority over the fixed?
I would be happy if the Rabbi could find an answer or direction to answer this.
Thanks in advance!