The problem is that your individual rasters are not aligned. So, when they are added the result is a weighted average for the values in the overlaping area. To solve this you have to use a merged vector layer whose extension must be used for producing individual raster. I used this procedure for testing it:

1. I created two vector layers where they are overlaped in some area:

2. I merged them and I wrote down its extension:

3. Using the extension to rasterize the first vector layer:

4. First vector layer rasterized:

5. Second vector layer rasterized:

6. sum_raster obtained by using raster calculator. You can observe that the sum is 20 in overlaped areas (Value Tool plugin).

The problem is that your individual rasters are not aligned. So, when they are added the result is a weighted average for the values in the overlaping area. To solve this you have to use a merged vector layer whose extension must be used for producing individual raster. I used this procedure for testing it:

1. I created two vector layers (both with the same value field = 10) where they are overlaped in some area:

2. I merged them and I wrote down its extension:

3. Using the extension to rasterize (by value field = 10) the first vector layer:

4. First vector layer rasterized:

5. Second vector layer rasterized:

6. sum_raster obtained by using raster calculator. You can observe that the sum is 20 in overlaped areas (Value Tool plugin). The values: 0 (black area), 10 (gray area), 20 (white area).