Perhaps it is a tradition from the era of waka (a traditional Japanese poem of thirty-one syllables), jiamari is oftentimes allowed for a vowel or a hatsuon (the 'n' or 'm' sound).
和歌の時代からの伝統であろうが、字余りがゆるされるのは母音ないし撥音が含まれる場合が多い。