The way the algorithm currently works can be summed up as "is there a section on your CV which covers the current date X years ago", for values of X from 1 to 5.
As of today (29th May 2014) this counts a blank for 2011, as your education is listed as finishing in January 2011 while your job didn't start until June 2011. So there is no section covering 29th May 2011 which means it's not counting towards your completeness score.
The good news for you is that your completeness score will become 25/25 when June 2014 begins.
I agree that the algorithm could be improved - what would make the most sense though?
Maybe it should simply be looking through the entirety of the previous 5 calendar years (i.e. 1st Jan 2013 to 31st Dec 2013, 1st Jan 2012 to 31st Dec 2012) to see if any section overlaps. But that would potentially allow big gaps while still giving a full completeness score.