Displacing points increases active voxel count

I am initializing a OpenVDB level set from another data structure, something like:

```cpp
while(...) {
idx = GetNextIndex();
pixel = GetNextPixel();
coord = openvdb::Coord(idx[0], idx[1], idx[2]);
vdbAccessor.setValue(coord, pixels);
}
```

The number of active voxels are 10498. If I displace all the indices by a constant amount(basically I have to translate them according to an origin), the number of active voxels becomes 12117. This translates to increase memory usage. What's causing this?

(my fundamental problem is that that original data has a concept of an "origin" and "spacing" between voxels, and I'm trying to recreate that in an OpenVDB grid)

Start at the openvdb::Accessor::setValue call shown in the issue and reproduce the active-voxel count before and after translating the Coord values. Compare the resulting grid structure and memory usage, then document the cause and how origin and spacing should be represented in an OpenVDB grid.

Written by the indexing model from the issue text.