From what i have observed this year, I have this fictitious theory.
Category 1 : for people who have got Decision Made in few months say 2-3 months after application is received usually get their oath late as there are much more people who have crossed the 12 month deadline and non-routine applications should get oath soon as they have waited long already and they dont want these category of people to fall down the mean statistics from 12 to 6 months citizenship processing time on the website. This in turn will put pressure on staff for expectations for next coming years.
category 2 : for people who have got Decision Made in say 4-6 months after application is received usually get their oath less than 2 month since they want to eliminate the pileup for oath ceremony by finishing oaths like a just in time process.
category 3 : for people who have got Decision Made in say 7-10 months after application is received usually get their oath less than 1 month as they have to finish the whole process in 12 month target.
category 4 :for people who have got Decision Made in more than 12 months after application is received usually get their oath very soon as they have been waiting and responding to RQ's and Fingers prints etc delays...