You may have just been very lucky when you got 4 Legendary gems from 6 Ubers, and very unlucky the rest.
Luck isnt the same as chance.Chance is fixed,with luck you can beat the odds.
There are 8 gems mentioned, so if these were in a bag every time you took a gem out, there is a 1/8 chance of getting a specific one.
You put the gem back in the bag,(you are not allowed to leave it out) and try again, it is still a 1/8 chance.You could try 6 more times and still not get the one you want.
Now supposing instead of there being only 8 different gems,they add a few more items to the bag.
Ubers drop things apart from gems.
Supposing they add one of each other gem, but none of Indicolite,the odds of getting this gem now drops to1/15.
Unfortunately, we dont know what the % chance of getting a particular item is,they dont publish the information, nor when and if it changes,or if its been adversely affected by a bug after an update to the game .
I notice the first event was KvK the second time was CvC, how long after was this,and the other times you are complaining about,were they comparable conditions ?