Firstly, it is commonly known and taught that branched alkanes are more stable than linear ones, and so have larger enthalpies of formation. But why is that the case? Are there some steric interactions at play?
Secondly, even if we just accept that relationship to be true, why are there instances where this relationship does not hold? Specifically, when I was exploring the data on NIST WebBook and I noticed that $\Delta_\mathrm fH^⦵$ is larger for 2,2-dimethylpentane than 2,2,3-trimethylbutane even though the latter is more branched than the former.