Steinbeck uses a lot of different "registers" in Cannery Row. "Register" is a fancy word for what kind of speech or writing is appropriate when (check out a definition here).
You guys know this. If you came in to school and greeted your teacher with a slap on the back and a "What up, dawg?," you'd be speaking in the wrong register. And possibly in detention. The characters in Cannery Row speak in a pretty casual register, even when they probably shouldn't. Think of the soldier on the beach at sunrise who asks the watchman, "Why don't you take a flying fuggut the moon?" (14.5).
But what register does the narrator use? Let's take a look:
This is no fly-by-night cheap clip joint but a sturdy, virtuous club, built, maintained, and disciplined by Dora who, madam and girl for fifty years, has through the exercise of special gifts of tact and honesty, charity and a certain realism, made herself respected by the intelligent, the learned, and the kind (3.1)
Here, at least, the narrator is using some slang—"fly-by-night cheap clip joint" aren't exactly words a university professor would choose. Yet the rest of the sentence is a little more formal: it's in a higher register than the characters' dialogue. Could you imagine Mack saying that Dora has "tact and honesty, charity and a certain realism"?
Of course not. He just says, "There is one hell of a woman" (23.63).
And then, sometimes, the narrator really goes off the deep end and starts spouting off like some guy in the Bible (which guy in the Bible? Get thee to the "Shout Outs" section):
Our Father who art in nature, who has given the gift of survival to the coyote, the common brown rat, the English sparrow, the house fly and the moth, must have a great and overwhelming love for no-goods and blots-on-the-town and bums, and Mack and the boys (2.2)
So the narrator goes all biblical on us up there, but then he sneaks in some stuff, like "no-goods" and "blots-on-the-town" that bring us right back down to earth. So, why all the switching of registers? Well, we think that it shows us we can trust this narrator. He may be a smart guy—but he doesn't think he's too good for Cannery Row.