It's also common for unions to agree between themselves about coverage of particular occupations. The optimistic view is that it's because one union can better represent that occupation, and the cynic's view is that it's because back in the day both organizations got together and carved up the territory.
As always, the truth probably hides somewhere in the middle.
Gee, back to the good old days of demarcation disputes--where unions jealously held "their ground", ostensibly to maintain work for their members (read, numbers are power)--just one of things that led to a widespread distate for unions in general among the general public, and led to gross inefficiencies which were well up amongst the reasons for governments privatising enterprises that were union strongholds. The very reason behind indivual workplace agreements, with workplace flexibilty, which led to efficiency gains overall.
That said, I would support the ALEA against the likes of Paul Howe's lot.