Why is the maximum adiabatic temperature found for an equivalence ratio a bit higher than 1?
I actually found the answer in this 2011 paper, here is a quote from the abstract:


  The characteristic rich shifting of the maximum adiabatic flame temperature from the stoichiometric value for
  hydrocarbon/air mixtures is demonstrated to be caused by product dissociation and hence reduced amount of heat release.
  Since the extent of dissociation is greater on the lean side as a result of the stoichiometry of dissociated products, the peaking
  occurs on the rich side.