Interesting question.
If you have 2 apples and you add to them another 2 apples, there will be 4 apples. it is a fact that you can see with your own eyes.
But, in the math the things can look different than they look in real. If you 'play' with numbers and not with apples, you can get this:
2 + 2 = 4
1.7 is approximately 2, but:
1.7 + 1.7 = 3.4 which is approximately 3
1.999(9) is approximately 2, and:
1.999(9) + 1.999(9) = 3.999(9) which is approximately 4
And according to the math the number 3.9999999999999999999999999(9) can have unlimited number of 9's and in fact it is very very close to 4, but can never reach 4.