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

Single ended class A amps don't so I'm going to assume you're talking push-pull class A. A class AB amp is usually said to clip when the demands of the waveform exceed the voltage and/or current capability of the amp circuit or power supply. Usually the power supply is the major limiting factor. 
A P-P class A amp is roughly like a class AB amp, but the bias is set WAY higher. Instead of a few milliamps just to keep the devices on in their linear region, the bias is cranked up to cover the entire waveform. Once a 100 watt P-P class A amp reaches it's bias limit, one side of the push-pull goes completely off and the other side conducts beyond it's bias. That's a p-p class A transitioning into class AB. That's actually how high bias class AB amps are designed to work. The result of the p-p class A amp behaving like that is massive amounts of head room. As Erik suggests, loads and designs strongly contribute to this, but that's the general reason why. 

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

If one were a Mosfet output and the other a BJT (bi-polar) and both were well engineered push/pull and driving into a heavy load, such as the Wilson Alexia which has an EPDR (equivalent peak dissipation resistance) of just .9ohm!!!! at 65hz.
Then the 100w BJT amp would drive it better than the 300w Mosfet.
As the 100w BJT "could" in effect give out 800w into that .9ohm load where the Mosfet would probably go down to less than it’s rated 8ohm 300w output.
Cheers George

Sounds like you are talking about two SS amps, so probably from Pass but that is just a guess.
To your question, the Class A amp will likely have a stiffer power supply and more capacitance.  Also, IME, I like the sound of Class A better since to me they sound more fleshed out than the best AB amps I have heard and so may sound more harmonically complete when pushed.  A last observation based on experience is that really good Class A amps do not get strident sounding as Class AB amps can when they are starting to get stressed, the Class A amps simply run out of steam - at least that is how the two different sets of Lamm hybrid monos I owned used to behave.  My 300 wpc Class A Claytons have yet to run out of steam, even driving my new lower-efficiency (85dB) speakers.
To George's response, my listening has resulted in a preference for bi-polar output stages vs. mosfets.  I believe some designers use mosfets to try and emulate a "tube sound" but to me they do not control the output as well.

There are a few examples...So as far as Headroom.... First an 100 watt amp that is true pure Class A amplifier won't play louder than a 300 watt A/B amp... Pure Class A means that it will not switch out of Class A from First Watt until clipping.... So the amp is a Class A amplifier... In this sense,  A  100 watt vs a 300 watt amp is a no brainer as far as how loud it will play...The 300 watt will have 6db more headroom. On the other hand,  there are several amps today that are sliding Class A,  They are biased to run Class A for a percentage of their output.  Threshold and Coda did this quite a bit.... I think of the Aleph 3 or 30,  they are rated at 30 watts class a, but in reality are more like 125 watts or so,  but they play their first 30 watts in Class A before switching to A/B, after 30 watts they play the balance of their power in A/B. 
I hope this helps,  Tim

Not having any credentials such as a manufacturer or scientist, I always refer to the basics.For me it adds even more confusion, because we're talking about a music signal heard ultimately, by ears.

http://www.electronics-tutorials.ws/amplifier/amplifier-classes.html

I took a couple of analog/digital classes in the early 80s. An instructor that was an audiophile would blow the students minds  with endless physics/science  theory behind audio. It was way over my head!

I did become aware that most of us really are listening to just a couple watts of "linear "power.