Why do more noise hits show up in the 2dh than in 1dh for this event?

If you take a look at the similar work going on at LBL (see Dieter's page and click on "Pattern Recognition" and then where he says they tag the merged particles), you may say (as Dieter did :) "it looks like there is less noise hits in 1dh than in 2dh.

It is true! (for this event). Looking into this, I found that a few of the unused 2dh hits could be eliminated by prohibiting one- and two- pixel hits (safe enough!), and by fixing a (small) bug that had to do with the interaction between the algorithm that cleans up noise at the TPC bottom and the removal of electron sequences (see the old description of the overall algorithm).
But there is still more noise hits in 2dh for this event.

Let's see where those unused hits are:

Here is a plot of y vs x of the unused hits in 1dh (red) and 2dh (green). Note that: