A common mistake in upgrading ROMs is mixing it with a different variant. Samsung Kies sometimes can be dumb enough to give you a different version, or will tell you that an update is not available. The bad thing is, if they proceed with flashing manually through Odin, sometimes they realize it too little, too late.
The Galaxy S2 has a lot of variants, including those American and its sub-variants (S2 LTE, Skyrocket HD, Captivate 4G, etc). Among these, 2 are the most common ones that were distributed locally - the i9100 and the i9100G. Whoa! One's got an extra letter, so it must be newer right?
Yep, the i9100G only started distribution around Q3 last year compared to the other one that started April. Comparing the two side by side, you won't notice any
difference at all... same 8MP camera, same Super AMOLED+ screen, same generation Gorilla Glass protection, same physical dimensions... but when you go into Settings or its technical specifications, this is where it all boils down:
Samsung Exynos SoC
TI OMAP 4430
The two devices have different internal body parts after all, not to mention 2 of their major components. The i9100 uses its own Exynos chipset, while the i9100G has a Texas Instruments-made counterpart. The same story goes for their GPU. If someone can recall, this particular PowerVR chipset is the same one that was used in Galaxy S. Rumor has it that last year, there was so much demand for this smartphone that Samsung started to run out of parts. So they got a 3rd party to manufacture additional parts.
Having said that, one can conclude that flashing the right ROM is indeed very important. My previous assumption was that if the S2 is white, it must be the G variant. That assumption turned out to be false, as there really are white i9100s out there and black i9100Gs as well. Heck, there's even a pink version for both these two! The best way to check what your S2 model is by going in "Settings" and "About Phone". Benchmarking apps like Antutu and Quadrant also shows your hardware configuration in detail.Source