I am guessing that he means that when he is done writing what he wants to write for this go-through, it is compiling every file, but other times it is only compiling 2 or 3 files.
The catch is the *.h files. Any change in a .h will require that every file which includes that .h (and I think all files which include THOSE files too) needs to recompile. Essentially this means any change in a Cv___.h requires a complete rebuild, but changes in Cy___.h require only all of the Cy___ files and a handful of other files to also rebuild.
It is because of this that I generally advise people to plan your program in advance, think of all functions & Variables that you are going to need and declare them all, generate the bare bones of each function, but don't tie it into the code yet. Compile your "long run" and check that there are no issues in game (at this point everything should work exactly as it did before your code writing). Once you verify that, THEN you go back into the code and start making it do something.
I personally prefer that while I am making the Bare Bones functionality I also include the text displays so that I can have new fields I created be used somewhere to ensure that the text display comes out properly, even if it doesn't actually do anything yet.