How `erl -make` does its work?

How `erl -make` does its work?

I found files won't be compiled in following situation with erl -make.
Compile file a.erl
Copy an old version of a.erl from other directory
Compile file a.erl
a.erl not compiled
Then I tested it on OS X, a.erl was compiled successfully.
In above two situations, the only difference is that the 'last modified
time' in OS X was changed when I copy the file with cp command, while not
changed in Windows without command.