Molecular orbital diagram of CO and charge localisation

[MO diagram of CO](https://www.chemtube3d.com/orbitalsco/)


My question concerns the interpretation of the Molecular Orbital of CO. I think I find it clear how you build it but I have some concerns about how you rationalize it.


Particularly, from what I read the reason we envision carbon as being partially negatively charged in CO is that the HOMO of the molecule lies closer to the carbon orbitals. However, from the MO diagram, it can be seen that the p-oxygen orbitals are closer to the HOMO than both of the orbitals of carbon. 


My question is: what we use as a benchmark to judge that the HOMO is closer in to energy carbon: are we looking at average energy or something completely different and I completely misinterpreted the argument?


