How students actually learn English...some theories?

At my uni, the students generally switch teachers for the second semester of the mandatory first year that they have to study English.  Except this semester, it was a bit of an exception for me.  3/9 classes are the same students as last semester, where it had only happened to me once in the past years that I've taught here. 

And in those three classes, it seems that they are actually better at English than the other classes that I've taken over from other teachers.  Like, their average scores on homework and tests are higher than the other classes.  I was thinking about why that appeared to be the case and came up with the following theories:

1. I'm just a better teacher than some of my colleagues.  I know this is for sure the case in some instances but I've taken over classes from some excellent teachers so this can't be true 100% of the time.  

2. My students just seem better at English because they understand my rules and instructions and feedback I give them after getting used to them the entire last semester.  The other classes are still figuring stuff out. 

3. The students are less shy.  They know me and I know them so they don't feel scared to speak up in class.  And they know I won't make fun of their mistakes or ridicule them in any way which makes it a safe kind of place to give an answer without fear. 

4. Luck of the draw.  This could definitely be part of it, but actually, one of the classes that's doing really well this semester was one of the weaker ones from before, so perhaps not so much.

Anyway, it is probably a combination of the first three things.  What I do know for sure is that I really like having students for the entire year.  I think it's hard for the teachers when students compare you to the last one they had.  And it's better if you can just start the semester off on the same page, instead of having to go through that whole adjustment period as in any new class where you're figuring out the students and they're figuring out how you run your class.  And I like the idea of building relationships over a year instead of just a few months.