Antenna peaks apply to a particular direction (in 3D) around the AP. The patterns are not homogeneous, and vary a bit (and sometimes a lot) from one antenna to the next.
So since the peaks for each antenna do not typically align, the peak of the average pattern for all antennas in a band can be quite a bit lower than the peak of the individual patterns for each antenna in that band.
Note that total peak gain is the sum of this "peak of average pattern" plus MIMO gain (3dB for 2x2, 6dB for 4x4).
The actual difference depends on the specific patterns and number of antennas.