Why chrome download medium & small size image at the same time?

[riddickhou]

As the responsive image, when I set the chrome to cell phone mode with screen size 340 X 640 px ratio 1, and monitor the data in the network, the medium & small size images download at the same time. Why would that happen?

The code is as below.

<picture id=”banner”>
<source media=”(min-width: 750px)” srcset=”images/still_life-1600_large_2x.jpg 2x, images/still_life-800_large_1x.jpg” />
<source media=”(min-width: 500px)” srcset=”images/still_life_medium.jpg” />

</picture>

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