Tan on my back but pink on the front

I went tanning for approximately 25 minutes, front and back, outside. I used SPF 8-tanning lotion, which left me tan on the back but more pink on the front (the sunburn is in its latter stages due to the constant itching). My arms are still pretty white. Was there something I did wrong to lead to the multi-colored areas on my upper body?

PS - I played some baseball games this weekend and now my arms and neck have gotten more tan, MEANING that it looks almost as if I never tanned earlier in the week. I'm a little upset.