The asphalt that is part of RAP is old and aged primarily through exposure to the elements.  Compared to virgin asphalt, it is stiff, subject to premature cracking and fatigue failure.  Simply put, its high-temperature characteristics have increased, its low-temperature characteristics and fracture toughness have decreased. Technically, when used in a mix design, the BBR S(60) increases, and them (60) decreases leading to thermal cracking in the finished pavement.  Also, the DSR G*sin d increases which lead to fatigue cracking.  The mixes with RAP also tend to be stiffer and harder to lay and roll correctly.  On the positive side, the RTFO G*/sin d increases which lead to improved rutting.  It is all of these changes that must be addressed when you use RAP.  Simply put, when you add the old, aged asphalt to the mix, its properties must be counterbalanced with the virgin asphalt (and rejuvenator when appropriate) used.