Quite a lot of services send both… and the text/plain version is completely different. They used to be the same many years ago, but then whoever is in charge of changing the email template only changes the text/html variant, and keeps the text/plain content stuck in the old times. It'd be almost funny if it wasn't tragic.
It is, although practically I don't really mind as my mutt is configured to filter text/html through elinks, so I see the content just fine. It's those other annoyances like tracking links that span a dozen lines that annoy me more. :-(
