basically, the concept of organizations is about to map the real-life (organizations - companies - business corporations) which can have multiple peers/machines which is participating in Blockchain network,
and such thing doesn't mean they didn't share the same ledger....
in matter fact all the organizations with all peers behind this organizations sharing the same ledger and once you submit any transaction from any of this peers and the transaction is committed and done successfully, the rest of organizations with all peers will sync with the new transaction and get copy of whatever happened in the ledger...
but keep in mind this should be happen in the channel scope,
channels is about to separate ledgers, each channel have it's own specific transactions / data separated form other channels
so if you have 2 channels let's say (Ch1 , Ch2) and each channel have 2 organizations (Org1, Org2) and each organization have 1 or more than 1 peer: all peers inside all organizations in channel (Ch1) will have it's own transactions and data and will not sync or share it with channel (Ch2)
summary:
organizations share the same ledger and get copy from any transaction and have it's own copy of data
channels didn't share ledgers and each channel have it's own separate data which is not shared with another channel
hope this point is very clear now for you....