Actually there is a plan in this madness. There was a list posted a couple of years ago in the form of a pdf.its so stupid that they are separating them as east and west . the ones we got are 191 and up but the phx based ones start in the 500's . is it because they have different engine types? are the rest of the A321 deliveries with the iae engines? and are they only gonna be flown by phx crews?
In the narrow body Airbus fleet it goes as follows:
319 East CFM powered, 700 series tail number
319 West IAE powered, 800 series tail number
320/321 East CFM powered, 100 series tail number
320 West IAE powered, 600 series tail number
other types:
757 non etops 900 series tail number.
737-300 were all supposed to be put into the 300 series tail number,
737-400 400 series.
All etops aircraft regardless of type are a 200 series tail.
The E190 was put in the 900 series
and now the IAE 321 is put in the 500 series.
With the new 321 deliveries, you would think they could have combined the order and created a common type as far as the engines go.