Should I justify an average GRE score with a good excuse? I received a 4/6 (59%) for my GRE analytical writing score. I would have scored higher had it not been for the fact that I type on an alternate keyboard layout (i.e. not QWERTY) and had to use QWERTY to type the essays. Before taking the test, I looked into getting approved to use my alternate layout, but it seemed like a good amount of time and paperwork would be involved, so I didn't go through with it. 

Should I mention this in my graduate applications? For reference, this is for application to United States PhD programs in physics. My GRE verbal / quantitative scores and GRE subject test scores are all much better (>90%).
I don’t think you need or should mention it.

First of all, 4/6 is not a bad score, even for a PhD application. Moreover, Writing is probably the least important section of the GRE test, at least for a physics major.

Second, the professors who review your application would expect to see other more meaningful indicators of your research potential, and thus, explaining why you don’t have a higher score will make you look petty. In fact, try not to talk about your Writing score because the reviewers might not care about it at all, unless you mention it.

After all, the rule of thumb is to focus on the strength of your application instead of explaining the weaknesses.