Possibilities of upgrading downgrading Stirgeman stuff ?

Heya all,

i managed to get the lvl 20 stirgeman pants.
So i decided to try to upgrade them.

i know that there is a chance (10%?!) that the item will be destroyed while the forging process.

I took the risk and upgraded my Pants. they went from lvl 20 to lvl30.
on my second upgrade attempt the Pants didnt improve and were still lvl 30
on my third attempt, the pants were upgraded from lvl 30 to lvl 70.

Now im a bit confused, how are the possibilities that my pants will reach lvl 90 after the next upgrading attempt??
I hope someone can help me.

thanks in advance for your replies