Exactly right. Different custodians have different mandates to "lend" out stock, it's in the fine fine print. They earn a small fee but it all adds up.
Say you were pessimistic and wanted to sell. Your broker would say ' terribly sorry Mr Atom you don't own those shares. But I'll get onto Cayman Hedge Shorters Inc and call them back for you so we can sell them for you.'
In practice this would only happen if your volume was significant because broker would probably scramble borrow a few shares for you (without necessarily telling you) to cover for small volume.
Worst possible scenario - shares go way up and poor risk management / credit issues "borrower". Those shares never come back. Opes Prime an example.
SGH Price at posting:
76.0¢ Sentiment: Hold Disclosure: Held