G15 (and SX50) Flash Questions

I have found that when taking flash shots with my G15 and my SX50 they both often choose an ISO of 640. This is the same whether using the internal flash or with an external 270EX flash unit.

Most of my flash shots are close range (less than 10 feet) so I find that I can get a better picture by manually setting the ISO lower.

I had two questions:

Firstly, why does the camera choose such high ISOs?

Secondly, how come I get the same bright images when I set the ISO manually to 100 even though both the exposure (1/60) and the aperture (usually widest) are the same whether the ISO is automatically at 640 or manually set to 100? Does the camera boost the flash output by 6.4 times to compensate for the different ISO or is something else going on?