Why is this character field column not merged when sorted in ALV output?

Hiya,

I've got the following output from my ALV:

As you can see the 'Net Ord' column doesn't seem to get merged even though it is sorted.

The column is a 13 character field defined simple by netwr_c(13).

There isn't NO_MERGEING in the field catalogue.

Any ideas on why it isn't being merged?

Thanks in advance!