First of all when you say natural base 6, are you talking about the roots only or was the entire head virgin? And what was the quality of hair? Porosity could attribute to what you are saying, whether the hair was aged, coloured or just neglected. Texture may also come into play.
When I think about brightness I personally think about light science. With that darker colours are duller, lighter colours are obviously lighter. Warmth tends to also be brighter then cool colours. If her hair was coloured was her roots warmer then her ends or vice verser. Was her hair coloured, sunbleached or rather even? Did you apply roots first or ends first
You've also diluted the base colour by half. I was always taught the roots were the "Heat Spot" so they tend to process quicker. I assume they were also virgin which would mean they would have to break base. As for your ends, they may well be porous (the hair is aged at least, I don't know if it was coloured or not as you have not said) and especially as the mix on the roots has half the reflect, I can see why they would be brighter.
Also this may seem like an odd question, but the water added, was it warm or cold? Warmth may have made the colour start processing ever so slightly.