A building permit comparison can help to shed some light on the issue:
In the past 30 or 40 years the city of Atlanta has never been near the top of this list, and now it's been #1 for the past two years. Also of note is that Atlanta is much smaller than the counties it's being compared to (Atlanta is 132 sq miles; Gwinnett is 437 sqmi, Cobb is 345 sqmi, etc), so the density of building permits in the city is pretty impressive. Finally, a building permit in Atlanta is more likely to represent a large multi-family development.