The Shimano 105 shifter for the front derailleur has 4 different positions: L-trim, Low, T-trim, Top.
The L-trim and T-trim are there so the rider can trim the derailleur and keep it from rubbing when in the "big front-big rear" and the "small front-small rear" combinations, respectively.
The Shimano dealer manual has detailed instructions on how to adjust the front derailleur. If you follow it carefully, you'll have a perfectly adjusted derailleur.
Shimano : Dealer's Manual : Front derailleur