The obvious answer is that front shifting is a lot more tolerant than rear shifters, especially for doubles. All it needs to do is pull enough cable to get to the limit screw for the large ring with a full click. If that requires more slack than you are used to so be it as long as it shifts. I'm using a 105 9sp triple left shifter on a 105 11sp double crank with a 105 11sp front derailleur. I shift up and it goes to the big ring without problem, I shift down and it falls down to the small. It has never derailed the chain, and it is a 10sp KMC chain so even that doesn't match. It just doesn't seem to matter. I have never been able to get my Chorus 10sp to work as well as this does.
Is Tiagra any different in front? I never understood why they made this one line somewhat incompatible with the others.