Yeah, they get that because we tend to type how we speak in person, and when we speak natural English, we use contractions like I'm, isn't, don't, can't, etc. When we add "have" we turn "I should have" into "I should've" - Unfortunately, this, when said outloud, sounds similar to "should of", and therefore billions upon billions of English-speakers make fools of themselves daily by writing (and saying) "of" instead of " 've ".
It's funny, my Japanese friend and I JUST had a conversation about this grammar topic YESTERDAY. Amazing. Anyway, carry on, you internet goons you.
P.S: Trevalkin I had no idea English wasn't your first language. You're really good! Wow. I hope I can be that fluent in my second language someday.