See Encode::MIME::Header for a further explanation. Also, RFC 2047 has the exact format and even a rationale for why it has to be encoded differently from the MIME email body.

The biggest concerns are, BTW, a lack of information during processing of the headers and backwards compatibility with systems that only support 7bit encoding for headers. It's a header that tells what kind of MIME encoding the body will use. If your headers are encoded using the same method, you have a chicken-and-egg problem. RFC 2047 gives a 7-bit clean escape sequence to state the encoding in the individual header, and both servers and clients expect this method of escaping for non-7bit headers.

This is a bit of a messy workaround, but it allows your MIME-encoded email with MIME-encoded headers to pass cleanly through SMTP servers that know nothing of these RFCs and be handled properly by the sending and receiving clients anyway.