96 jetting

[Knobbly]

I have a 96 with a Nexxus silencer(full exhaust?)after a few hiccups I have it running quite well, if a little flat.
The main jets were right-122.5 Left-125 which seemed a little strange so I fitted the unstamped jets supplied if the kit I purchased that are of an unknown size. The rest is std.  I now find in my recently purchased parts book that the main jets should indeed be as supplied as std(although there are MANY other variations)
 
My questions are-

1 Why are the main jets different for each cylinder?

2 Why has no-one mentioned this disparity but always mention one size being good\bad\rich\lean etc?

3 Is the often mentioned 127.5 main jet the best size jet for my set up?.......especially if performing 4 below

4 Given ignition advancers are like unicorns and the machined 5 degree advancer conversions appears to have dried up too, is there anything preventing me from performing a DIY drilled mounting hole to slot conversion after some simple geometry to advance the ignition?

Thank you to those able to offer help

[Knobbly]

I have just remembered that the needle is set to one clip slot richer!  Packing washers are often mentioned on here for a richer midrange, am I correct in assuming that single slot needles occur mostly in the USA where the anti-meddle legislation was more stringent.