I have to preface, that I do rating, not systems, but what you say is only partially true.

If you mean that for instance, you are looking at all the horses that won last start and then take it as gospel that they will win again, yes it's back-fitting.

On the other hand if you are looking for horses that won their last start in similar class, same distance and preferably under the same track conditions, then you are doing genuine research.