My left STI shifter has 5 positions for a double crankset?
As @Pete said in the comments, front derailleur shifting needs a partial-shift positions. That stops the chain rubbing on the front derailleur as it moves across the cogs at the back of the bike. As you shift across the chain bends coming off the chainring and the path it takes through the front derailleur changes.

So normally there will be an extra part-shift on each gear for a double chainring (four positions) and the middle of a triple has one each side for a total of seven. Five doesn't make a lot of sense, but without playing with your exact shifter it's hard to know where and how big the shifts are.