AFAIK when you use "send to Kindle", Amazon takes your epub and runs it through Kindlegen (or whatever Amazon's current equivalent is) to create an azw3 on their end. They are not rendering the raw epub - it goes through a black-box conversion before you see it. Thus, all bets are off as to how the "epub" will look on your Kindle. (Indeed, Calibre does a better job converting to azw3 than Kindlegen! That's why we use Calibre when creating azw3 files at Standard Ebooks.)
I won't believe Kindle supports epub until I can transfer an epub via USB cable and have it available to for immediate reading. Anything else is Amazon "supporting" epub with very big air quotes.