David and Duncan,
Thank you for replying. David, in answer to your question about what tool I am using, I have tried weighted sums, cell statistics, raster calculator, and the sum tool, and each gives me the same result. What I am trying to achieve is exactly what you described: "the sum of a number of rasters results in a new raster, with each cell value being the sum of the values for that cell within the input rasters." The cell size is the same for every raster (1000 x 1000, in this case meters).
Duncan, thanks for your suggestion. To see if this was the issue, I reprojected several rasters and set the snap raster environment setting, but after summing the rasters with weighted sums, the end result was the same as before.
Attached are three rasters if either of you feel like you have time to take a look at them. I am relatively new to GIS, so let me know if it would easier if I attached an ArcMap document with these rasters already added instead.
Thanks,
Jeff