I just got 50/50 perfects for a third time, but still no achievement. I made sure I finished the levels in order as well. So the only thing left I can think of is getting a perfect on the last day on the first try.
10k 29/03/09 - 100k 17/07/10 - 200k 02/09/11 - 300k 02/02/13 - 400k 20/11/13
Achievement #1000: Seek and Destroy - #10000: Fully Evolved - #20000: Data Security
Overachiever: 55 points - 16 Staff Clubs - C/F Clubs 10-5-1