Why does using emoji reduce my SMS character limit to 70?

When I start typing a message in the Messages app on my iPhone, it shows the character limit at 160, but as soon as I type a single emoji character, the limit drops to 70.

It makes sense that using double-byte characters would cut the maximum number of character in half (80), by why is it less than half (70)?

I assume there’s some extra header being sent that indicates the message is encoded differently. I’m curious what the actual contents of that header is and why it takes up 20 characters (or 10 double-byte characters).