Mainely because below Tc two phases exist, each with its distinct minimum.  (see for example "Thermal Physics" by J C Lee 2-nd edition, Section. 12.9 )

There are two equations (conditions) of critical points that should be valid. (Of course they are not directly connected with these minima.) Any case the mathematical (thermodynamic) equations for critical points are well-known and presented in most thermodynamics books and monographs. Of course one should look on these equations (of course in good book!) and all would be clear enough. You can see, e.g. Munster "Chemical Thermodynamics" and others.