The wood elves will get those buffs for any battle that counts as a forest map, they don't need to actually be standing in the forest on the battle map.
Sometimes maps that don't particularly look "foresty" count, so keep an eye on your unit buffs to see if you have them.
As you have found out, archers shooting into or out of a forest is generally a no-no. A lot of people hate this as it's against the lore, but they are already pretty unbalanced for campaign play, so allowing them to shoot unimpeded in a forest would probably be broken.
Seriously? I thought they got those buffs while inside the forest... Well thanks for the explanation.