Violet centiles are a score up to 100. If you're on the 50th centile that means you're bang on average. Any scores between the 16th and 84th centiles are considered to be within the average range Anything below the 16th centile is below average, and anything above 84 is above average.
As an example using height (which is easier to visualise!!), if you were on the 90th centile for height, it would mean that you're taller than 90% of women, and only 10% of women were taller than you (i.e. that you were quite tall).
My midwife told me that it bears almost no relation to birth weight- your bump size is influenced by loads of things e.g. how much fluid you have, the thickness of your placenta, and just how you carry it. They just track it to make sure your bump is still growing, so it's kind of a score to compare to itself, but doesn't really mean anything else. Your chart is also personal to you according to your height, weight etc, so two people's bumps may measure the same in cm but be on different centiles.
My bump is also on the 90th centile, which I found quite funny seeing as the midwife told me that in the same conversation as she was telling me I should be having extra growth scans to make sure my baby's growing enough as i have a low BMI 