What part of the site are you talking about? I looked through this video that’s under the “View Projected Population Growth” link…
…but it doesn’t have that 510k number.
I have a feeling that number is extrapolated from Census projections. People interested in population stats have been getting hyped up about Raleigh breaking that 500k barrier soon (though recent data released just the other day suggests that it may take a while).
We have a whole thread to talk about how much Raleigh is really growing, by the way, if you want get into the weeds.