There have been a number of threads on the topic, but their titles aren’t exactly indicative of it really. Also they get intermingled with the AI “luck” factor threads. Here are a couple, but there are more, just look for threads that seem to have a “rantish” kind of title and you’ll probably find them 
I don’t know the exact reasons, but it started from the move to Unity, and shifting to the same base as console. Something hear caused an issue where gem spawns were matching almost every time. I believe because they weren’t overwriting their own colour, or dual spawners weren’t overwriting any of their possible colours (eg blue/brown wouldn’t overwrite blue or brown, even with the other colour) but I’m not 100%, I don’t use gem spawners much myself, and never really have. So to counter this the spawners were nerfed.
As to why they can’t go back? Honestly I’m not sure, but I’m guessing it has to do with how the code interacts between Unity and AdobeAIR (which mobile is still on). Unity never had the AdobeAIR code, and it might be difficult to replicate that code in Unity, hence why we can’t go back to what it used to be. Maybe we could go back to what console had/has, but I’m not sure how that code would interact with AdobeAIR and Mobile since it never existed there in the first place.
These are all my own speculations however.
EDIT:
I’m really interested in seeing these figures when you finish them, I’m a sucker for numbers Which reminds me, I’ve got my own AI Kerb numbers to continue to gather on…
EDIT2: Looks like @Mithran sniped me with a similar post as I was typing.