, I think I'll throw in my theory as well.
Ok, as you are working along, you are going to unavoidably twist the yarns together as you work. Nothing wrong with that. But, as sometimes happens, one yarn is being wrapped around the other. Which takes up more real estate, or yardage. Or if one color is sitting on the other color then that takes up more real estate also.
So, my theory is..... the pink is wrapping around the red and/or sitting atop the red, hence using more pink as you go along. Which actually, if you think about it is a good thing. That way you don't have to try to join the next ball or skein of pink yarn in the same place as the red, thereby, spreading out the joins for a cleaner finish. But the next time it might be the red using more. It all depends on which yarn is sorta like being the dominant, straighter one and which one is wrapping around the other.