Guys my distant relative retired from Nokia(gurgaon) in Jan 2014 (I recently found out he had been working) When asked about the why nokia delays updates He told me this (He told me a lot of technical stuff but i didn't get it, anywho i'll be writing the jist)
Lets see the science behind the updates
Amber Update dates-
1st 620-Aug 15 =,2013
620(India)- 5 Sep 2013
1st 520-16 Aug 2013
Lumia 520(India)-24 Sep 2013
Moreover some country variant such as Israel 520,Colombia 520,Colombia 720, Costa Rica 520,720, the Black updates have not come
+ Colombia's country variant of 820,920 haven't seen the amber update till now.Yes 920!
When giving updates Nokia divides the world into regions, For example Scandanavia, European Union,UK,Eastern Europe,Gulf, Middle east Asia, Brazil, Central and Southern America, Canada, US, NZ, Australasia, Asia Pacific, Japan+Korea,Russia, Far East,etc The actual number of divisions is greater, as each of them has further divisions into country variants
such as MEA1 which incldes one country India, MEA3 which includes UAE, Bahrain, Kuwait,Qatar , sometimes Jordan
So if MEA3 is updated all the MEA3 countries will get the OTA update.
The(OTA) Update process depends on three factors
1. The Market of the phone
2. The Market of the phone in the country.
3.The Market of the country.
The higher and the lower in the above categories get update first. For ex: Middle east market is yet to grow, so it will get update first so that people can mouth to mouth publicize the phone and to increase the market share, similarly the scandanavian market would get update first, as 1 in every 5 person has a windows phone over there (to reward the hooked customers).
Similarly new launch devices would get update first(i.e come with the update) to increase the market share, the devices with the highest share would get updated first like 920,520.
When Microsoft began with the gdr1 update all the 620.520,720 hadn't been launched so all came with portico, when gdr2 came in july the number of 620 had then outnumbered other low end devices like 520,720 but by the time amber was ready 520 was already the most popular handset, that's why 620 got amber first, but gdr 3 in october 620 was still the 3rd most popluar device (of the x2x series) in many countries (520>920>620) By december 620 had dropped to 5th positon (520>920>720>520>620>1020>625>820>822). And Remember the growth of WP market in india is halting. So 620, is a average phone in an average WP market in an average WP growing country. All the factors delaying the update here. Plus there's more
The time it took so long for update on black for 620, It's an average popular lumia phone.
It has features like NFC, Clearblack LCD(instead of IPS),relatively low battery potential, No inbuilt FM antenna, wifi -802.11 a,b,g,n,
which makes it unique than other low end lumia devices, hence desiging a update specifically for the mobile required more time when compairing other low end lumia.
And finally Nokia had been busy being bought by microsoft + other devices such as 1320,1520, x, X+, XL which required desiging a almost whole new OS.
+ a range of new devices for WP8....He told there are gonna be atleast 3 before june but he doesn't know nothing about them. Also a debate on a firmware update too (8.1+nokia specific)
Black update on whole was delayed. (GDR3-October, black- Jan).
He said that i may have to wait untill the end of the month for black