MPG Depending on Driving Styles

[Magenta Sunset]

"The only thing I can think is that when revving the engine more freely rather than laboring at low revs it actually makes it more efficient?"

which is a generally recognized driving technique. (dropping a gear going uphill before the engine labours is a good idea in that respect)
also, keeping as high a gear as possible without laboring or juddering the engine/trans is another one.
skipping a gear changing up if say in an urban location is another.