Why did the Human Reaper look Human?

Question

I'm guessing that the real answer here is "because the writers changed their minds," but is there an in-universe reason for why the proto-Reaper at the end of Mass Effect 2 actually LOOKED human? Mass Effect 3, especially Leviathan, clearly established that

Reapers were made from the harvested races but that they were all based on the crustacean-like design of Harbinger, which in turn resembled the Leviathans.

There are a few size variations from this norm, but otherwise all the Reapers look fundamentally the same. At the end of Mass Effect 2, however, you find

a giant Reaper being constructed, and it looks like an enormous human body:

So why did the Reaper at the end of ME2 look like this? Did I miss some explanation of that, or did the writers just decide that having each Reaper have a totally unique design wouldn't be worth it? And if it was a change of design, why did the Dark Space Reaper fleet in the final shot of ME2 already feature the consistently Sovereign-esque design, instead of a race-by-race design for each ship?

Answer 1

According to Mac Walters, the lead writer of Mass Effect 3:

The exterior of the Reapers does follow a similar pattern, an efficient design for the purpose they were created for. However each Reaper is created from a unique species, and as we saw at the end of Mass Effect 2, the core of each Reaper is designed in the likeness of that species.

So what the player fights in Mass Effect 2 is only the "core" of a Reaper - and IIRC that makes sense because it's much smaller than the complete Reapers we see at other points in the games.