The problem with page 177 is the image, which if you solely focus on that and ignore the hex labels looks like a singular building with one, Level 3, roof. However, the example text and hex labels are correct.
The hexes the Locust are moving across are labelled Level 2, and the text says the Locust is at Level 2. This means that the Locust is moving on the roof of a Level 2 complex, rather than actually being inside what (in the image) looks to be a single building. It's only when it gets to Hex B that it actually moves inside a building, and so incurs the PSR. The Marauder, on the other hand, is entering the building from the very first hex it moves (which both the image and text actually back up), which is why it needs to make two PSRs.
I can't do much about the image, but I can clarify the text slightly in a future reprint to make the Locust's movement clearer.