I ask my german friend a math question:
I expected my German friend to answer it as this: $$1.000 - 1 = 0$$ $$1{,}000 + 1 = 1{,}001$$ but instead my German friend says this: $$1.000 - 1 = 999$$ $$1{,}000 + 1 = 2{,}000$$
All of my German friend said this, which isn't correct. Why do so many Germans think that 1.000 - 1 = 999? He said he's good at math so why does he thinks that 1.000 - 1 = 999?
When writing numbers, Germans use the periods and commas the way Americans use commas and periods, respectively. Germans write $1.000$ for our $1,000$ and $2,000$ for our $2.000$. Their $1.000-1=999$ means exactly the same thing as our $1,000-1=999$, and their $1,000+1=2,000$ means exactly the same thing as our $1.000+1=2.000$.
Wikipedia has an article on various national usages of the decimal separator and thousands separator. Americans use the period for the former and the comma for the latter; Germans (and many others, including most of Europe) reverse this.