Why does batter often taste sweeter than the baked product?
I think that SAJ14SAJ listed some good examples, and that these certainly contribute to the overall taste. But I think there is yet another one, whose effect is strong enough to matter: rising. 

Let's say that you coat 1 cm² of your tongue with a foodstuff. The more tastebuds are activated by a sweet molecule, the more is it evaluated as "sweet". First imagine your batter. It is dense, and coats all of the tastebuds on your tongue. Each of them is likely to register a few sugar molecules. 

Now imagine the baked cake. It has air bubbles in it. If the plane of a cake cut has X% air bubbles by surface, then only 1-X% of your taste buds come in contact with the sweet cake. And the concentration of sugar molecules in the solid part of the cake surface is the same as the concentration in the batter, so you have the same number of sugar molecules per activated taste buds, but less taste buds activated. As cake volume contains lots of air, you also get lots of air bubble area in a cut. The effect is reduced by chewing, but by the time you have chewed the cake very well, it has also been diluted by lots of saliva, so now you have a much better coating, but less sugar molecules per taste bud. 

By the way, this effect also occurs with salt in bread dough vs. baked bread, so I don't think there can be factors uniquely bound to the details of sweetness perception, as salt uses very different chemical pathways of being tasted.