Is a haiku really five-seven-five syllables?

Not quite. The rule you learned at school, 5-7-5 syllables, is a loose copy. Traditional Japanese haiku are three phrases of 17 morae, called on in Japanese, in a 5-7-5 pattern, and a mora is not a syllable. It is a smallest unit of timing, equal to or shorter than a syllable, in languages where vowel length matters. Where English verse typically counts syllables, Japanese verse counts on.

The counting rules show the gap. One on is counted for a short syllable, two for an elongated vowel or doubled consonant, and one for an n at the end of a syllable. So the word haibun is two syllables to an English ear but four on: ha-i-bu-n. The city of Ōsaka has three syllables and four morae, because its first syllable has a long vowel. A haiku by Kobayashi Issa contains 17 on but only 15 syllables. Going the other way, a sound like kyo may look like two syllables to English speakers but is a single on.

So what happens to the English 17? In 1973 the Haiku Society of America noted that English writers usually used 17 syllables, yet about 12 English syllables approximates the duration of 17 Japanese on. So 17 English syllables last longer than 17 on.

Even the Japanese masters did not always keep to the count. Bashō has a haiku with 18 on, in a 6-7-5 pattern.