Why no Ivan Spawn ? or why so less ?

I have come across Ivans in 1.8, I couldnt say for sure whether it is more or less - I even came across one at the Alien Camp which was a first for me!

I have killed 3 so far but they do seem to spawn a bit less which is good. Seeing Dr. Ivan every 30 minutes in the old patches was kinda lame. I actually get excited now when i get one.

Apparently the spawn rate is that of approximately 1 in 50 Zombies.

From experience, closer to 1 in 100 i think.

With a 6% chance of each Blueprint dropping is also the word 'pon the wind.

I mainly get Ivans spawn once I have farmed 40+ of his friends in an area... he then spawns to 'apparently' take me out... he fails every time