g++ option -MM -MF afile is not the same as -MM > afile if inside makefile

[Budi]

Why the result of g++ option -MM -MF afile is not the same as -MM >
afile if inside makefile with VPATH defined somehing?

The first is defected i.e. incorrect and incomplete as opened/printed
out by cat afile,
but so confusing is how come the process is not affected at all i.e.
it finish the job perfectly the same ?
Please help explain this behavour !