When I write my entries on lang-8, I always evaluate all the corrections I received before deciding which version I like the best. I often keep a final, edited and revised, copy of whatever I wrote; that way I can study it or use it later. So, don't think the students even see your corrections as "definitive". Its not like there was meant to be one correction only. Like Half-n-Half said, there are many different ways to say something.
Though, I must rant here for a minute: the Filipinos and Malaysians who correct my corrections incorrectly, and then argue with me about it when I provide hard evidence as to why they are incorrect, drive me crazy! I know my English is not perfect (I didn't catch the misuse of the past perfect that opened this topic, lol) but I have a master's in TESOL and English IS my native language. Don't mess with me. And if you do, at least submit to citations and references as proof that you, sir, are wrong.