Difícil vs difíciles

[Bobbert]

Listening to a language learning podcast, even though the focus was not on the use of difícil or difíciles, I did notice the teacher saying the following when giving example sentences:

Rules are difficult to understand
Las reglas son difícil de entender

I am questioning why did she not say son difíciles de entender?

Chileans are difficult to understand
Los chilenos son difícil de entender

I am questioning why she did not say son difíciles de entender?

How would you have said it?

Are both ways commonly used or is one of them incorrect?

Side note:
I think this teacher is from Colombia. I found it interesting to also hear her using the word “computador” instead of “computadora,” which I learned in Mexico, or “ordenador,” which I learned in Spain. This just shows how much variation there is from country to country and region to region that makes the Spanish language so rich!!