When you use 'Send to Kindle' to send an epub to your device, you are not reading an epub. In the link above, it mentions how Kindle converts epub to an Amazon format before allowing your device to read it. Amazon's formats on the whole are inferior, with poor rendering capabilities, and an automatic conversion means all bets are off in terms of what the ebook will look like.

Kindle will not natively support epub until you can connect it to a USB cable, transfer an epub using a file manager, and it does not get secretly converted.

I did not know that and does kind of explain one book with pictures that looked a bit weird. On the whole though, text epubs are totally readable as I get most of my books from non-amazon sites such as smashwords and email them to the kindle to read.