Other recent reports has 2018 GDP at 3.1%
It looks like they're using two different measures.
If you compare the size of the economy in Dec. 2017 vs. Dec. 2018 you get the 3.1% growth rate.
If you compare the total output in CY2017 vs. CY2018 you get 2.9% growth.
The latter is the number used more commonly historically but both are perfectly valid measures.