Please give me a simple explanation in this.

I disabled auto iso and took some too. The results are here.

Flash set to REAR.

First shot. ISO 200. The camera chose 1/8.

Second shot. ISO 400. The camera chose 1/15

Third shot. ISO 800. The camera chose 1/30.

Fourth shot. ISO 1600. The camera chose 1/60. I feel the overexposure kicking in.

Fifth shot. ISO 3200. The camera chose 1/125. There is an ugliness of flash here.