MySql 5.7, maximum varchar column length is 768?

Question

MySql ver 5.7.28, following sql fails with error: "Error Code: 1071. Specified key was too long; max key length is 3072 bytes". But when changing 769 to 768 then it works well. Why this happens?

DROP TABLE IF EXISTS `items` ;
CREATE TABLE IF NOT EXISTS `items` (
  `id` VARCHAR(769) NOT NULL,
  PRIMARY KEY (`id`))
ENGINE = InnoDB
DEFAULT CHARACTER SET = utf8mb4;