I have noticed that when I am 1 artifact away from being able to create something in the CT Lab that I won't get the last item for several days.
It took ~5 days for me to get the 50th space elevator, and about the same amount of time for the 50th warp gate.
and currently at 36hrs for the 100th Android Helmsman. (been getting 2-3 per day)

Just seems super coincidental that the drop rate reduces to nothing soon as your close to getting enough....

yeah, almost like how video poker machines are programmed to almost never let you win....

it's a matter of the brain's perception of randomness (it's all in your head)
I seriously doubt Dan threw in code to make the last arti pull very very very very rare