Bottom line on top, the difference is due to the fact that in the bistatic case, the angle of incidence does not match the angle of reflection.
The standardized RCS definition is given by
where R is the range, Es is the scattered field, and Ei is the incident field.
It is easiest to discuss RCS within the context of bistatic radars, because the monostatic geometry is a special case of bistatic radar when the transmitter and receiver are colocated.
Now, there are 2 standard coordinate conventions. There is the forward scatter convention and the backscatter convention. Both Radar Toolbox and Phased Array System Toolbox use the backscatter convention, so I am going to limit my response to that.
Depicted below is the backscatter coordinate system.
In the case of the bistatic geometry, when the transmitter and receiver are not colocated, the incident and reflected rays are not the same.
In this system, the scattered field is expressed in x3, y3, and z3 coordinates. The fields are related by the Sinclair matrix as:
The left side is the output scattered field, the S matrix is the Sinclair matrix, and the right most matrix is the reflected field. The above can also be expressed in terms of linear polarizations (H and V) and circular polarizations (left circularly polarized [LCP] and right circularly polarized [RCP]).
The scattering matrix S can be used to infer a complex RCS scattering matrix that has a similar form to the Sinclair matrix:
The components of the polarization scattering matrix are related to the square root of the radar cross section such that the elements are
where it is recognized that 
is a complex number that has amplitude and phase. If the receiver and transmitter are colocated as in the monostatic case and the medium between the target and receiver/transmitter is reciprocal (reciprocity theorem), the Sinclair matrix is symmetric 
(
) and the geometry simplifies to that shown below. In this case, the incident and reflected rays are the same.
If you are interested in learning more, I suggest taking a look at the book Radar Cross Section by Eugene F Knott et al.
Additional documentation for relevant features:
where Gt and Gr are equal putting Smin = Pr will give max range as 
. While in Bistatic Radar Tx and Rx antennas are placed apart. Thus if alpha (bistatic angle, sum of incidence and reflection angle) is small then bistatic RCS is similar to that of monostatic RCS.
