The design may call out a specific material and how to manufacture it, say make this part out of 6061 Aluminum, and. mill it in this way. You can look up the standard properties of that material. But depending on how that material is worked, cut, bent etc, you would expect in come cases some changes in the material properties or just sensitivities to fatigue. E.g. if you cut a sharp corner into that metal, fatigue will increase in that corner over the life of the part, so maybe you specify rounded corner - but how much it makes a difference you won't do necessarily a whole specific detailed study on it unless its a critical part or corner. If it's milled, if the machine that day has a duller bit, maybe it heats up the part more, and the material properties shift a bit more due to the heating. All these and are controlled in the process, but are still little things lead to variation, and maybe the controls fail one day and some slightly different aluminum makes in into the process.
For critical parts on the aircraft, there is the backup of inspecting for aspects like accelerated fatigue just for variation of manufacturing, of environmental contributors, etc. It's a lot like security defense in depth - many layers of protection for lives (and aircraft industry trustability linked to profitability too) at stake.