High ISO Azure is so quiet - how so?

Azure Kingfisher (Ceyx azureus) at Jourama Falls today. Image so-so, but much quieter than would usually be expected, given ISO of 3200 (f8 1/500 Canon 7D 300L f4+1.4x). High ISO means noise. But tip doing rounds has it that Canon auto ISO produces less noise than same ISO manually selected.

Here's the unaltered full frame

Full frame denoised, sharpened and overcurved

Cropped, denoised lightly and sharpened

Cropped, more denoised and sharpened

And second look at quiet (for ISO 3200) Azure. Seems a good tip.