Level 6 dye turned level 4 brown instead?

I had hair that was bleached to a level 9 blonde. I dyed it navy blue, kept it up for about seven months, and then tried to fade it for another six months. Last night, I used the Punky Colour Off Hair Color Remover Kit with 10 volume developer and got it back to a pretty consistent level 8 blonde. Today, I used the Wella Colorcharm in 6N Dark Neutral Blonde, but the color is nowhere near a level 6. It's more like a level 4 or even a level 3. It's not necessarily a bad thing, since it's pretty close to my root color and it will fade, but I'm curious why this happened. Was it just a bad bottle, or is there something about the process that could have caused it to become darker than it was supposed to be?