One theory says that uniforms became lax in and after the mid Heian period and there were examples of 'ikan no sokutai' and 'noshi no sokutai' where ikan or noshi was used as an alternative of sokutai, and the particle 'no' was omitted from the former phrase, which was misused and then became established.
一説には平安中期以降、服制が曖昧になり、衣冠や直衣を束帯の代用とした「衣冠の束帯」「直衣の束帯」という例があり、前者の語から接助詞の「の」が省かれて誤用され、定着してしまった結果であるとされる。