While a developmental editor should pick up most inaccuracies in content, they work with the understanding that spelling, grammar, and punctuation errors will be addressed in later edits. In other words, your developmental editor will not read your manuscript with the aim of making it 100% error-free. Your copy editor will do that as part of their service, leaving the proofreader with the task of identifying rogue errors that have previously been overlooked.
It’s important to note that the copy editor reviews the text before typesetting. In contrast, the proofreader reviews the (often printed) proof — that is, the book as it will appear in its final format, after typesetting.