Before 1901 anyone could call himself/herself anything (except for a noble name). You just told the vicar what you wanted to be called and that's it - and you could change your name any number of times.
A Swede's true identity was his/her patronymic (e.g. Andersson/Andersdotter), the rest was just decoration. Even today Swedes - except for the tiny fraction of the population that makes up the nobility, some old clergy and old smithing families - have no particular feeling for their last name; it's one of the most popular pastimes here, to change your family name (last year more people than ever before changed).
The reason the last boy added Lind (which means linden tree/lime tree) was either that he served as a soldier (all soldiers were given surnames by their regiments), that he moved into town as something other than a servant or factory worker (e.g. to work in a shop or become a craftsman) or that he somehow got himself a bit of an education which enabled him to climb a step or two on the social ladder. Lind was certainly a soldier name, but not exclusively so (Lind is by no means an exclusive name, 15454 people have it for their family name in Sweden today).
As to why he chose Lind in particular - well, if the reason was that he joined the armed forces he didn't have much of a choice (it was mostly the regiment's decision). If he chose it himself, he probably just liked the sound or there was a linden tree back home or something like that.
Read more about Swedish naming practices:
Names
Ingela