Hola Steve

The subjunctive is explained in slightly different ways in different sites/books. 

When we say the subjunctive is used with "aunque" if that information is "shared" by the speaker and listener we mean that that bit of information is not new for them, so it is something that has been mentioned before or is somehow known by the speaker and listener (maybe because they heard it from someone before?). So here are two contrasting examples, both correct:

1. Aunque mañana llueve, nosotros vamos a la playa.

2. Aunque mañana llueva, nosotros vamos a la playa.

1 is most likely to be used by someone who wants to present that information (tomorrow is going to rain) as a piece of new information.

2 is most likely to be used if they´ve already heard on the news that tomorrow is going to rain, therefore this bit of information is "known/shared" by the speaker and listener. 

Here is another site where they explain this usage using the words "información conocida" - if you go straight to the contrasting pictures they show at the bottom with the examples "Aunque engorda un poco, nos encanta...- Aunque engorde un poco, nos encanta...", they are explaining the same thing. 

https://gramaticas.marcoele.com/elefante/subjuntivo/subjuntivo-presente/subjuntivo-presente-aunque/

 

Saludos

Inma