There are a few achievements that have number in the popup event that are being removed. It said that you got the top tier because they didn't differentiate the tiers in the event when they first put them in. You got the correct 500k tier. I will poke them and let them know this is one of the achievements that need tweaking on the popup. Once changed, all tiers will have the same popup to avoid confusion like this.

Oh no no no! Thank you for letting us know it is still going on with some!