And they were so easy to find once the book was in print....

This happens often when proofreading onscreen. When printed out locally the errors are more visible. When coming back from the printers, glaringly obvious.

My reasoning is that the more temporary/interim/changeable the less importance you put on the task of checking. It is psychological imperative. How final is the stage? The change of texture and subtle changes in the font certainly factor-in too.

Things I notice? minutia, detail. But then I am an engineer, and programmer, detail errors can be binary in their consequences.

Proofreading requires more grammatical &/or syntactical knowledge of language, not my forte! And certainly not on my own texts!