Hi Sarah, I've moved this post into AD, since it's not a class question per se.
This is very interesting. It's actually rather common, but usually the difference in print size is bigger than this. Usually 4x6s and 8x10s would be printed on the same machine at the lab. However, it seems like that might not be the case? If they were printed on different machines, and one of the machines was running a bit dodgy that day, it would be the most common explanation for this.
Have you asked the lab about it?