Most people who say they are 100% something, are most likely wrong. When you watch a few videos on those heritage self tests, you will know that no one is 100% one 'tribe'.
I can trace my ancestors back 6-8 generations, we've never left the Netherlands, but when I do a DNA test I do get the UK and Scandinavia in the mix as well.
I can say that I am 99% or something Western European.
Saying you are 100% Italian, because your grandparents were 100% Italian, you will probably get Southern European in the mix as there were plenty of people travelling across the Meditterean Sea. Americans who say they are Irish, also probably have more Western European in the mix.
Fun video is when the Try Guys do the DNA test. Eugene is absolutely sure his family his 100% Korean. The shock on his face when he hears there is also Japanese and Chinese in his family.