This isn't so much about math, as the fact that mathematical models of a material phenomenon can be erroneous.If you take 2 quarts of water and add 2 quarts of ethyl alcohol (grain alcohol), you will NOT get 4 quarts of mixture, but a bit less.
Once again, whether the formal conclusion that 2+2=4 applies is a matter of observation and testing. Sometimes it does. And sometimes it doesn't.