If I say I have been to McDonald's all across the country, that does not mean I have been to EVERY single McDonald's restaurant. Just various ones across the country
Sure, but I would argue that you're speaking imprecisely then and using a cliche or phrase that isn't technically accurate at best, and is actually misleading at worst. And when requested for clarity, you should say, "well almost all." Either that or we have different definitions of what the word "all" means[1].
If someone said "I have been to all the states in the US" would you expect that they have been to AK and HI?
[1]: The MW definition matches my understanding: https://www.merriam-webster.com/dictionary/all
“All McD’s” - quite precise; literally all the McD’s.
“McD’s all across the country” - a much looser group, implies various McD’s spanning roughly coast to coast (hence “all across the country”), but not necessarily including the westernmost and easternmost McD’s - just a decent distribution of west-to-east, perhaps with no giant gaps (like missing the whole Midwest).