105 Shimano 10 speed shifters - compatibility with 11 speed rear derailleur
This is to do with the different pull ratios employed by the different systems.
In your case 10 vs 11 speed Shimano.

http://blog.artscyclery.com/science-behind-the-magic/science-behind-the-magic-drivetrain-compatibility/

If you look at the table in the link - it says the 10 speed levers pull less than the 11 speed levers but the rear derailleur ratios are also vastly different.