Whenever someone is arrested, certainly in Scotland, a new picture is taken as part of the custody suite process. This is added to the database alongside their previous pictures.
Rationale for this is that historical crimes are commonly reported and for an emulator board, you would need a picture from that time period, etc. When you do a search to compile an emulator board, you are always given the most recent picture unless you specifically request the older ones.
So that probably counts for a fair whack of that 18mil when you consider that people with 180 previous convictions are commonplace, certainly where I work!