That's because the word is 伸ばす. The 伸 kanji for this word has a reading of の.
As you might already know, Japanese has three distinct writing systems (not counting romanizations): ひらがな, カタカナ, and 漢字{かんじ}. It is preferable when possible and when it is normal to use 漢字 in your writing because, while ひらがな and カタカナ are syllabic characters, 漢字 convey ideas and don't have a fixed syllabic value in Japanese.
What this means is that 漢字 have multiple ways of being read, and their readings are mainly grouped into two categories, which are 訓読{くんよ}み and 音読{おんよ}み. There is an additional category which is 熟字訓{じゅくじくん} when a character or set of characters have a reading outside of these two standard categories. Use of 漢字 in writing helps distinguish a word for its meaning rather than its sound.
Say, for example, that you see いって written down, without much context. How would you be able to tell if the word was meant to be いって (て-form of 行{い}く, to go), or いって (て-form of 言{い}う, to say)? They are pronounced the same way. 漢字 help discern what the true meaning of the word is, as it is written. Therefore, you should try to use 漢字 when you learn it, so that your written Japanese is more clearly understood.
I should add a note: Some words are more commonly written in かな than in 漢字. As you study, you'll learn which these are, and why this is the case. You may, for example, use the 漢字 for 居{い}る but more commonly you'll see it written as いる instead. That's another thing to keep in mind, and something you'll pick up along the way.