Jinbocho Station - mystery tunnel

The other day I got the Hanzomon Line from Jinbocho Station, possibly for the first time ever, and my first impression was how dirty the Hanzomon Line platform is, at least compared to other stations in Tokyo. The picture below gives an idea, though it doesn't really catch the dusty atmosphere.

The above-ground station entrances are a bit dingy too. On the other hand I've used the Toei Shinjuku Line from there a few times in the past and don't recall it being unusually dirty.

The other thing I noticed is that towards the eastern (Oshiage) end of the platform, the tunnel suddenly widens a little (you can just about see where this starts at the centre left of the photo above), and at the eastern edge there's what looks like a side tunnel branching off northwards from the Oshiage-bound track. It certainly looks like it might be wide enough to be the stub of an unused tunnel:

Close-up:

At first glance it looks like it was intended to be a connecting tunnel to another line, but the only logical destination would be the Toei Shinjuku Line, which runs roughly parallel. However the Shinjuku Line has a different gauge, which would render any connection somewhat pointless.

I can think of two possibilities for this structure. Originally the Hanzomon Line was intended to take a northerly route around the Imperial Palace, and the phantom stub tunnel certainly points in the right direction for this. However it wouldn't explain why the tunnel gets wider on the other side, and I don't think the line planning would have changed after the construction of this station. Alternatively, as the Shinjuku Line tunnel is located more-or-less above the Hanzomon Line tunnel, this layout may be just a side-effect of the requirement to support the Shinjuku Line. The Tokyo Metro website has a schematic diagram which shows the approximate positioning of the two platforms. I'd put my money on this latter possibility, but haven't find any information one way or the other, either in English or Japanese.