A contradiction in 13.2.1

​My understanding of the second sentence of the first paragraph is that read committed mode never sees "changes committed during query execution by concurrent transactions". For example let's assume two transactions, A & B, and the following:

• A started before B
• B starts before A commits

My understanding of the second sentence means that if A commits before B then any updates made by A will continue to be invisible to B because B's snapshot was before A committed. Now if I'm wrong about this then there is no contradiction forthcoming.
The final sentence of the first paragraph is where I find the contradiction. It says: "Also note that two successive SELECT commands can see different data, even though they are within a single transaction, if other transactions commit changes after the first SELECT starts and before the second SELECT starts​"

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So the mental model I've built based on the first four sentences of the first paragraph is that when a transaction starts in read committed mode a snapshot is taken of the (database) universe as it exists at the moment of its creation and that it's only updated by changes made by the transaction that created the snapshot. So for successive SELECTs to see different data because of updates outside of the transaction that created the snapshot is a contradiction.

Now my guess is that I'm thinking about it all wrong so if someone in the know could shed some light on where/how my mental model breaks down I would appreciate it.

Regards,

Dane