As far as I know, Callan is a trendy alteration of Callum (click link). Boys' names ending in -an have been very trendy in the last 20 years, which would explain the high prevalence of Callans. It's common around here (Perth) too.
♦ Chrisell ♦
All we have to decide is what to do with the time that is given us. - J.R.R. Tolkien.