2V voltage drop like a LED
The Maxim MAX7221 provides constant current source and sink for every segment of every digit, set by the resistor Rset that must be connected between the Iset pin and VDD. First thing to check is if that resistor has been connected, and if so, whether it is correctly calculated for the desired current per LED.

Regardless of the number of LEDs in series, as long as the supply voltage is sufficiently higher than the forward voltage of the combination of LEDs, the current through the segment (or the dot) will be constant. In other words, the MAX7221 output pins will automatically provide different voltages for the single-LED and the double-LED current paths. 

As LED intensity is related to current through it and not voltage, if any difference in intensity is observed is most likely due to difference in the optical behavior of the light-pipes used for the digit segments and the dot segment. 

Adding an extra masked out LED on each of the dot connections will not expose those LEDs to 160 mA as stated in the question, but to the same current as each of the other segments is set to, via RsetM/sub>. Thus, if it provides any satisfaction, one can go ahead and add those extra LEDs in series without a problem.

Despite what another answer claims, the datasheet for the LED display clearly states:


  Forward Voltage Per Segment Or (DP)       4.0 (2.0) 


This means the forward voltage of the number segments is 4.0 Volts typical, while that of the dot segment is 2.0 Volts typical. Thus, the contention that the number segments are 2 LEDs each, and the dot is a single LED, is perfectly valid. It doesn't affect intensity when using constant current driving, through.