Thanks for reporting Kami! That's a rounding issue of the percentage displayed. The uncommen crit has 1.6% while the rare crit has 2.4%. It's not quite doubled but still more. And both get rounded to 2% when displayed ^^ Added it to my todo list to display those chances with a higher precision.
Then 2 blues or 4 greys are still a bit better than a yellow? This would be inconsistent with the other potions afaik.
In terms of crit chance yes. All other stats are doubled. It's mainly because I don't want the crit chance go up that easily. So if you're lucky enough to get a rare one early game the impact is not that high.