Fractional alignment

The commath package provides a highly questionable implementation of \abs. Don't load this package unless you absolutely have to. (Aside: You do not have to for the example at hand.)

Instead of loading this package, I suggest you

• load the mathtools package instead of the amsmath package. The mathtools package is, for all intents and purposes, a superset of the amsmath package; and

• issue the instruction

  \DeclarePairedDelimiter{\abs}{\lvert}{\rvert}
  

  in the preamble. This sets up a proper implementation of \abs{...}. If you need to auto-size the vertical bars, simply use \abs*{...}.

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*Addendum": More about how commath implements \envert (\abs is just an alias for \envert): If \envert{<arg>} is invoked without an optional sizing argument (a whole number between 0 and 4), the following code gets executed:

 \!\left\lvert#2\right\rvert

It's the \! ("negative thinspace") instruction that messes up the symmetry of the full expression. Either write \abs[0]{<arg>} to execute \lvert#2\rvert or, more safely, don't load commath at all and go for a cleaner definition of \abs. The definitions of \abs[1]{...} thru \abs[4]{...} are also messed up, by the way, as are the definitions of \norm and \norm[1] thru \norm]4]. While it's possible, in principle, to correct the faulty definitions of \envert, \abs, \enVert, and \norm, one is better off going for a setup that avoids these complications...