It might be useful to think of it this way.
The second violins, for instance, are a section; they normally play together as a section. Many scores don't specify how many second violins are in the orchestra; they just expect there to be as many as the orchestra has.
When the composer needs this second violin section to change its behavior and split into two parts, the composer uses the label "divisi." This indicates that these two notes aren't being played as a double stop by each musician; the different notes are being played by different musicians.
"unison" is used to return to the "default" behavior of a string section: playing together as a section.
On the other hand, traditionally, orchestral scores listed exactly how many woodwinds would be playing. The first and second flutes are two separate "sections," and the expected default behavior is that they would be playing separate notes. Each musician has his or her own part, that shows only his or her own notes on it.
In the orchestral score, to save space, these two parts are sometimes combined. "a2" is how we indicate that we are changing this default behavior; it lets the conductor know that the same notes are appearing in both flute parts.