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...

'Headroom' can have a lot of meanings in the audio world...

At system-level you may think extra db from the average listening level the system can play before non-linearity/distortion, for example. This does involve the amplifier, but all other elements also including acoustics.

Headroom for amplifiers can mean extra power capability above rated, typically for shorter peak periods. But that's not all the possibilities for the meaning.

If you are using speakers that reach down to low impedance at some frequencies (as mentioned by georgehifi previously), power supply / output stage current capability and output impedance may be way more important that rated output power. First Watt Class A is a great example of that type of amp that can drive low impedance easily w/o fatigue on difficult loads.

But it all depends if that's a factor in your particular speakers/system/listening style. Really. An apple may not be the right fit for a person who is looking for an orange...

Unfortunately there's no general rule of thumb figure, it's all situation-dependent.