tl;dr: Upper case letters can be represented in "alphanumeric" mode, which uses 11 bits per two characters (5.5 bits per character, but padded if the length is

tl;dr: Upper case letters can be represented in "alphanumeric" mode, which uses 11 bits per two characters (5.5 bits per character, but padded if the length is uneven). Lower case letters are not included in alphanumeric mode, so the QR code has to be represented in byte mode, which uses 8 bits per character.