There are major differences in word choice, style, and general feel of text written in manga. And not all text is dialogue, sometimes there are narratives, not to mention "あらすじ" (summary)
For example, compare Naruto to say, Kochikame (こちら葛飾区亀有公園前派出所)[There's a reason it's called こち亀 LMAO!!!)...