The only cause of this is having Document Grouping turned on.

When performing a search, the system takes the first group of documents (Based on the Max Results Returned per Query value) in order of Document ID, and then it will apply the document grouping.

Thus, if your max results are set to 100, it will find the first 100 documents that match your query in the database and then group them together. You could then have as few as one document (if everything grouped) even though there are still more documents available. When you select More Available, it will grab the next 100 documents that match your criteria, group them, and then display the hitlist.