Since document gathering is usually performed by automated means, the searchable database is rebuilt regularly. This means that new or updated documents are often indexed shortly after coming online, and that dead links are removed in timely fashion. No two keyword indexes work quite the same. The way you initiate a search may differ dramatically from keyword index to keyword index. While some keyword indexes are case sensitive by default, others allow you to choose whether or not a search will be case sensitive.
Many indexes ignore words of three or fewer letters.
And some search only indexes of keywords, while others search the complete text of pages. The quantity and quality of results you get from individual keyword indexes vary every bit as much as the interfaces and search forms themselves. Search results often lack precision. Quantity of results is not generally the problem. But precision is another story entirely, and easily the most disappointing factor of all.