Hi Karen

We've added a hint to that specific snippet, to consider the English "some" as singular in Spanish. We also use algún/alguna in singular when we even have a reference in the plural form, to mean "one". There is a note inside the lesson that talks about this specific use. Have a look here. 

Check the note saying "But be careful" inside the lesson, right after the first set of examples.

Saludos

Inma