In the education system, I used SEE (MSS). Most of my deaf friends used ASL. I never knew anything about SEE, MSS, ASL, etc. To me, it was just... "sign language". So, I never knew if I was signing right or wrong. I just signed the way my teachers signed and my friends signed.
After graduating high school, I went to college and had ASL interpreters. I didn't even know they were ASL interpreters. Yes, they signed differently than the interpreters I had in school while growing up, but i still understood them.
It wasn't until I went to RIT when I finally realized what ASL, PSE, and SEE meant. I was making a request for interpreters for my classes when they asked me what kind of interpreter I wanted. I just told them "sign language". They still kept asking. That's when it was determined that I was PSE.
Turns out that without separating the two, I ended up learning SEE and ASL at the same time and mixed them up.
It wasn't until 5 years after than when I found out what MSS was until I ran into one of my elementary school interpreters and she told me about it.
I understand ASL from most people, but there are different kinds of ASL. I think that how ASL is used is similar to how spoken English is used... among different groups. So, who signs the correct ASL?
In your case, you probably had a person who didn't sign ASL properly or did sign ASL properly (and you weren't used to it).