This is my thinking process: 

Ok. Further investigation shows me the quagmire that this cubemap area really is. There seems to be no standardisation in terms of cubemaps. Some maps have facets that are rectangular as opposed to being square.... The one I used has a cubemap whose LxW proportions are not an integral number of facets, it's slightly displaced to the left... (I've contrasted it to make that visible). Imagine the fun and interchangeability we could have if we all followed some standardised format!!