Question about Master Seeds & key generation

If I buy a hardware wallet, say a Ledger Nano S, I’ve read that I get given a master seed which is the key underlying piece of information I need to recover my private keys if that Nano S gets stolen or lost , etc. 
I’m confused about one step in that process though: how does my master seed in the Ledger set up create different private keys that work on different blockchains (i.e. Bitcoin and Ethereum and Decred, etc)? Why do I not need a master seed for every blockchain I’m using?