yeast-quantity

[PD]

At the moment I brew 19litre beers and ales and use dried yeasts like Notty and S-04. Always had a good quick and easy ferment of my brews.
However I've come across several recipes for 23 litre brews which I am altering for 19 litres. Several of these show using 2 packets of yeast per ferment...Why ?
Surely one packet is enough ?
There seems nothing different with these recipes to what I've currently made.

 

[farmer brown]

Humm.. Maybe they are higher gravity stronger ales?

 

[31bb3]

I've only ever used 1 pack of yeast for 23L brews up to around 1050 ish only I'd only use more if it was a high OG then I'd make up a starter to kick start the yeast