In a dense index, why might an index entry not appear for every search key value in the file?

In database indexing, a dense index is an index where there is an index entry for every single search key value in the file. However, in the context of this question, if an index entry does not appear for every search key value, it is likely because the index is not truly "dense" as traditionally defined but rather acting more like a sparse index. In a sparse index , the index entries do not appear for every search key value but only for some of the key values. For example, a sparse index might store pointers only to the first occurrence of each search key in a block or a page, not every occurrence within the file. This reduces the number of index entries and thus the storage overhead, but it means that not every search key is directly represented in the index.