official ROM seens as custom?

[webrod]

Hi,

I upgraded to 4.3 my i9305.
However, there is a known issue with the battery (and other issues BTW).
My battery is empty after 4 hours only (without using it!).

So I decided to downgrade to 4.1.2 => not possible, it says 'unsupported version' !
So I downgraded to 4.1.1 => OK but it says custom/custom!
I bought 'triangle away' and now it says 'custom/official'
and still:
1) waranty void
2) count:1
3) under android it says 'modified' so no OTA etc

Any idea why this official ROM is seen as 'custom'?
If I upgrade back to 4.3 then it's OK it says official.

thanks for your help,
Rod