Yes, matte paint (or matt I suppose) is, or should be, the same as flat. The thing to remember is that matte paint uses a texturing agent (similar to talcum powder) to give the paint its flat appearance. The agent serves to break-up the light dispersment at the surface of the painted item. For this reason, matte paints has to be
completely and thoroughly mixed or the paint will not exhibit its true matte appearance.
I can only guess that either the paint was not totally mixed or the agent in the paint has deteriorated for some reason (although I have never heard of this happening before). Additionally, as you dilute flat paints, you are also diluting the amount of flattening agent in a given sample. This can also account for a reduction in matte appearance.
I hope some of this helps.
Enjoy your modeling...