updating to IOS 9 always showing not enough space

Hey

I am updating my iPhone 6 to IOS 9 and I have around 2Gb free space and it needs 1.1Gb and always when it shows 2 mins left to finish downloading it never reaches 100% and it shows a message there is no enough space and try to free some space...

So what's wrong? I have enough space but why is it showing that? I tried it many times and always same issue