Well, then you have a lot of explaining to do with the coincidental neutralness of the bore being completely covered... That is just too large of a coincidence for me :p That's all I'm saying. I don't know if it was intended or not, if not, that's a huge coincidence that it just turns out to turn neutral with wide extension.
There is also the third possibility, it originally was intended to be completely neutral. Then somewhere towards the later stages of development decided that they wanted more bass response or even to make it so that the IEM is less fatiguing. Add in the bass bore to allow more warmth in.
The possibilities are limitless. IDK if the bore being filled was part of the initial design. I have doubts like you do, but the coincidence seems too large for chance.