why xperia m/dual firmware start with 15?

[bang]

anybody know
why the firmware numbers xperia m/dual beginning at 15 that goes beyond other smartphones firmware
but this phone release long time ago and still in 4.1

i take sample from http://www.flashtool.net/firmwares.php

XPERIA Z1 (C6902,6903,C6906,C6916,C6943)
14.2.A.0.290 Customized FR

XPERIA Z ULTRA (6833)
14.1.B.1.532 Customized FR

XPERIA SP (C5302,C5303,C5306)
12.0.A.1.257 Customized US

XPERIA Z (C6602,C6603,C6606)
10.1.1.B.0.166 T-Mobile US

XPERIA T (LT30P)
9.1.A.1.142 Customized TW

XPERIA V (LT25I)
9.1.A.1.145 Customized SG

XPERIA S (LT26I)
6.2.B.0.211 Customized MEA

XPERIA ARC S (LT18I)
4.1.B.1.13 Vodafone DE

XPERIA PLAY (R800I|X)
4.0.2.E.0.57 Verizon USA

SONYERICSSON XPERIA X10 (X10I)
3.0.1.G.0.75 Customized FR

maybe some member from xda forum can give us some answer here.

[Hampus · Accepted solution]

Each phone is different projects within Sony that use different developers for the software and the hardware. If a phone have a higher build number, it does not necessarily mean that it has a newer software than other phones. Another Sony model that have a build number that starts with 15 is Xperia L, which mean that Xperia M/M dual is not alone with the build number that starts with 15.

[bang]

roger !
but why do they use the number 15
why not lower.
but it does not matter
it is just a number

[Hampus]

There is no particular reason why the build number differ from the phone models. And unfortunately, I do not know why they have chosen the number 15 for these models. The only thing that you need to be aware of is if the build number for your model changes.