Many writers are unsure of themselves and for that reason lack consistency. For example, within the same article, essay or book, they might spell words or use punctuation in different ways. They might spell a name "Terri" on one page, while referring to the same person as "Terry" on another. Or they might use a period outside of quotation marks in one sentence and use it within quotation marks in another instance.
Even if a writer is not certain how to spell a word, use punctuation, etc. she should be consistent. The words should be spelled the same throughout the work, and the punctuation should be the same when the same circumstances apply. It drives editors, proofreaders, and worst of all readers crazy when trying to figure out how a word should be spelled, how the writer wants to use punctuation, etc. At least if a writer is consistent, those reading, editing or proofreading her work will be better able to figure out the intent of the writer, and if necessary, how to correct her writing.