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4.0, as expected, even seems aware it's a small code demo and some more insight https://chat.openai.com/c/68a51900-4d33-4d1b-bd14-247a739183...

Google Bard also gave a clear answer: https://imgur.com/a/8HGivir

"The animation has some randomness" - really? Where?

The expression involving Math.sin is acting as a PRNG. The vehicles have randomised sizes and speeds.