This has been explained in the news by a number of journalists -- I believe it is because, Palin used her yahoo email account to discuss "off-the-record" thoughts with various members of her administrative staff. However, her dumb ass didn't realize that by sending from a yahoo account to staff email accounts, makes those received emails subpoena-able for public review. So the folks requesting the release of email records, were also able to get inbound email records from staffers received from Palin's yahoo as well as emails sent from staffer personal email accounts that were inbound to Palin's non-personal account. What is missing from the entire data set would be emails from Palin's yahoo account to staff personal accounts and vice versa.
As one journalist I saw on the news discussing this issue, what's interesting is the stuff that falls between the lines, i.e. redacted or missing from certain conversations that might be referred to.