Varying tinned flavours compared with slow transition?

This feels like a very basic question but I’m strangely confused!
We’ve got a couple more type of kibble to try with our 3 year old Cockerpoo to see if they help reduce acid reflux. As advised everywhere, we move her onto a new variety progressively over 7 to 10 days. If these don’t make a difference then we plan to try moving to tinned, to see if it’s just kibble that doesn’t suit her.
We would move over to that over 7 to 10 days too. We are aware of and taking account of allergies over all of this.
But … then there are of course a number of different meat versions of the same tinned brand - turkey, lamb, etc. Often sold as mixed multipacks.
So why is the advice on transitioning slowly between different foods suddenly less important when it comes to different meat varieties from the same tinned food company? Is the meat source not the issue and it’s the other ingredients that it takes time to get used to? Or is moving between different meats not really a problem for some dogs with hardy stomachs, whereas with the sensitive ones it’s probably best just to stick with one meat variety anyway?
Hope you can see why I’m confused?! Thanks :-\