Can you give an example of a correct response (from one of the other indices) and an example of the erroneous response too? Also can you show the mappings for the index which is producing the errors.
I suspect you are hitting the same issue as described in this post: Sum Aggregation returning very small, unrelated values
If the responses and mapping are long then please paste them into a gist and link that here rather than filling this topic will large amounts of json.