I've just reached 30 reputation on my Stack Overflow account, and am now being shown a banner saying "Congrats, you've gained the privilege – vote up learn more"
That is great, except you get vote up rights at 15.
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If we only showed you that you gained a privilege at the exact point you had that amount of reputation, many people would miss the notification.
The system tracks when a rep threshold is reached and then notifies the user, once, of the new priv. The user then can dismiss the notification and it will never be shown again. Sometimes the rep levels match exactly, sometimes they are higher and very rarely they are lower.