This is because the first project (the one with the biogas tanks) is the one that officially removes CO2. So if you want to remove as much CO2 as you emit, then it happens through that project. It is checked and verified in accordance with all the current international standards and therefore they can also be officially called "climate compensation" and it is fairly easy to check and verify - you catch some gas and burn it off. When it is burned off, it is gone and you can not really dispute that.
The tree planting project, on the other hand, does not count as offsetting at all. Of course, it absorbs a lot of CO2, but Climaider does not count it. This is because IF something were to happen to the forest - a natural disaster, for example, they still guarantee that you have removed all the CO2 you needed, through the other project.
So the trees are just a bonus and something we do because forests and trees are important to the climate - especially mangrove trees like the ones they plant in Madagascar.