The percentages given aren't for "forest" or "marsh" (those are called "terrains"). Under each terrain, several "terrain features" are listed -- "light undergrowth", "shallow bog", "gradual slope", etc. -- and given game effects. Those features are the things given percentages, and they can apparently co-exist in a single area -- e.g., you have a gradual slope covered in light undergrowth. (Though some combinations would seem odd -- you have light and heavy undergrowth?)
Rolling those percentages for every 5' by 5' area, especially for an outdoor fight in an RPG, would be insane. D&D isn't the D&D Miniatures Skirmish Game; the PCs can and will leave the battleboard, without counting as lost figures. So what do you do when they run off the edge of your carefully crafted map? Start rolling for each square?
Of course, you don't have to roll 'em; you can just use 'em as guidelines.
The terrain stuff in the DMG is a really neat idea that is, IMO, messed up by trying too much to be compatible with the minis game.
The way the least little difficulty in terrain halves your speed, with no intermediate decrease, also annoys me. What, 2/3rds is too hard to figure out? You can use the same chart you do when figuring out reduction in speed due to armor or encumbrance.