How could 100 Watt class a has more head room than a 300 Watt amp Class AB

To compare meaningfully output power of two amplifiers (all external factors being equal) it is useful to know several parameters.
--the manufacturer's definition of the power output metric. It should be
   "continuous power", meaning the amplifer's delivery  is not time limited  and it does not overheat in the ambient temperature 
--what is the distortion 20hz-20khz
--what is the signal/noise ratio
--what peak output power can be reached at 2-4-8 ohm, for how long, at what frequency and with what distortion
The peak parameters are useful to know, as musical peaks may need 5-7 times value of the continuous output power.
  

you 'presumed' but apparently did not actually 'measure', a classic audiophile mistake. You said the 100 watt amp 'appears'.........means nothing in this scenario.

my "shoot from the hip" answer is that the one with the more robust power supply will have the most headroom and be capable of the most dynamic swings

dont know excatly what type/topology of amp you're talking about. I am sure that it would not be the case with some other brands/models. But it would generally be explained very simply by the fact that the AB amp has less power reserve, or the constructor is just fooling customers, or something else in the specs has not be taken into consideration; output impedance maybe...