What is "x & x" in keyword density checker?

Recently I measure keyword density of a popular technology site. I found
"x & x" keyword is highest.

x and x	82	7.34%
e and x	8	0.72%
c and x	7	0.63%
f and x	7	0.63%
d and x	6	0.54%

Code (markup):

What this means?

 

Those are possibly variables for something else, or you aren't filling us in totally

 

Don't worry about keyword density, it is not a big factor in ranking algorithms. Just set the keyword you want to rank for as the first part of your page title, repeat it in your content a couple times, then build links that have anchor text as the keyword you want to rank for.