This white spotting is usually caused by dehydration and not an effect of hydrogen peroxide. It may also be caused by the use of fluoride-based toothpaste as this blocks the absorption and action of the gel.
All of the best tips and directions are included in the institutions on the strips.
Most importantly It will even out, you would just need to continue the course of whitening strips and the blotching will even out, and all the teeth will whiten as they should also stop the use of a fluoride-based paste for the course of the strips. This can resume once the course is done :)
This does happen with some people and will even out in a couple of days with continued use of the strips.
If you have any concerns you can check also with your dental professional.