Why does semipermanent hair dye binto hair?

First I want to say sorry if my English is Not that great, I am Not a native speaker
I am currently using semipermanent hair dye and it is really hard to get rid of it..
I searched the web for some ideas how to get the color out of my hair and stumbled upon a theory I can Not explain to myself:
It says, that if you bleach semipermanent hair dye, especially blue dye, it will Brand the dye into your hair, making it harder to get rid of it.
As a chemistry Student I am wondering if this is true? And if so, how can it be explained, regarding the chemical reactions that are happening in this process?

My 2nd, shorter question is: how exactly does efassor work?
I tested the pH of it and it was about 13, what is very alkaline. Also, it does contain Strontium Peroxide and some Perborates, so I thougth it reacts similar to the bleach we find in Hair Salons?
What is the difference between efassor and regulär hair bleach?
And what is the difference between the use of efassor with water and the use of efassor with Hydrigen Peroxide?

If you Do Not know it, here is the ingredients List at the bottom of the package:
http://www.capitalhairandbeauty.co.uk/Images/Products/Default/large/297223.jpg

Kind regards