It’s difficult to judge the quality of editing without seeing the original manuscript.
If the writing is really clean, it’s easier to catch nearly all the errors. (No one can reasonably claim 100% error-free results.)
If the manuscript is a mess, the editor may be so busy fixing the major problems that smaller errors near the major problems are overlooked. (Literally overlooked — the eye skims over them and focuses on the bigger problem.)
These other factors may also affect the quality of editing in a published book: The writer was willing to pay only for a quick read-through; the writer specified that only certain kinds of errors were to be fixed; the writer rejected the editor’s changes; the writer revised after editing and introduced more errors.
