What kind of teaspoon is this?
Like dlb suggested it is a spoon for sugar. At least in Germany this type of spoon is not uncommon with sets of cuttlery.

Update:

For an impression of various forms of spoons check a picture search. I searched for "Zuckerlöffel" (German for sugar spoon) and found a wide range of different forms.

As for the specific form I don't think there is a special reason other for having a different looking  spoon that fits the design of the cutlery set.

Historically sugar spoons where of a complete different design than your everyday cutlery as sugar was quite expensive and not used everyday. There are even examples of sugar pots that have small locks on them so that the servants weren't able to nick some sugar if laying the table.