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

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. 

Timlub, sorry to do the math for you but 100 watts will gain 3db every time the wattage doubles.  200 watts +3db, 400 watts +6db.

You will be hard pressed to tell the difference in amp power when comparing most a really good 100 watt class A to an average 300 watt A/B.  Check the specs also for dynamic headroom.  That might be your answer.

@elevick 
You are correct,  I just threw out 6db because of going from 100 to 300, but yes, you would need 400 watts for the EXTRA  6db of headroom.
I do disagree to some degree on not telling the difference.  Not necessarily but could be.... I have heard a 40 watt amp that had a huge power supply that was so dynamic that if you had fairly sensitive speakers that you would easily believe that the amp was a lot of power... On the other hand,  regardless of power supply.  If you had a very low sensitivity speaker,  say 82 to 84 db and you liked listening at a decent level, ALLTHINGS BEING EQUAL  you wouldn't be hard pressed at all telling the difference between 100 watts and 300 watts. Of course, you didn't say all things being equal and I do agree that overall, I would prefer a 100 watt Pure Class A amp to a 300 watt mediocre Class A/B