Perfection v. improvement
Editorial professionals, be they copy-editors or proofreaders, work to improve the content they are presented with. Ask a dozen copy-editors to work on the same manuscript and give them all the same reference books (such as Butcher’s Copy-editing, The New Oxford Dictionary for Writers and Editors, New Hart’s Rules or Merriam-Webster and The Chicago Manual of Style), then read the twelve edited versions. You will find that no two versions will be exactly the same. Why is this?
They will have spotted a great many of the same changes needed in spelling, grammar, capitalization, consistency and the use of language. But their individual suggestions will differ, if only slightly, and across the manuscript there will be a number of variations. Each edited version will be an improvement but no two versions will be the same improvement.
As an editorial professional I aim at perfection, but writing is subjective and each revision of a text will find further improvements to make. There are constraints on everything and deadlines need to be met so, at some point, the work must stop and the improvements deemed sufficient.
Realistically, perfection is the aim and improvement is the reality.