Actually, It's more likely that only one tower is Under construction. The plan origonally called for the project to be built in phases. Emporis:
"According to the plan the first phase of the project will be completed by the year 2008 with final completion expected for the year 2011."
So this project didn't really have an early start, phase 1 was always planned to be completed by 2008.
America's Tallest UC
Nordstrom Tower, NY 1,775 ft UC 432 Park Ave, NY 1,397 ft UC
30 Hudson Yards, NY 1,287 ft UC Three WTC, NY 1,171 ft UC
Comcast Center, Philadelphia 1,121 ft UCWilshire Grand, Los Angeles 1,100 ft UC
Salesforce Tower, San Francisco 1,070 ft 53W53, New York 1,050 ft UC