Because the first 1.5 miles was run as a squad in 15 minutes.
How long did that leave you for the other 1.5 miles?
Funnily enough, at 55, I just turned in 14:13 for the fist 1.5 miles and 14:28 for the second. Quite pleased with that for an old fart. Wouldnt want to do it in a pair of Boots Combat High though - not at my age anyway.
Just off to do a quick three miler