This is related to the way E-ink Kaleido 3 screen works:
https://www.eink.com/brand/detail/Kaleido3
If the page is partially in color, any color content will be displayed in 150 PPI and B&W content in 300 PPI. If you have a colored background and B&W text on it, the text should be in 300 DPI as well, but I didn't have a chance to actually test it, so don't quote me on that.