Are you connecting to a flight that is operated by someone other than United? For example, if you were flying to Europe, you might connect through Frankfurt, and then a flight from Frankfurt to your final destination in Europe. The flight from Frankfurt to final destination could be operated by a partner airline ("other airline") such as Lufthansa.

Edit: to answer the other question about why the date of purchase would have changed. The receipt and the date of purchase shows the date the new ticket was issued (so today), as mentioned above the original issue field still contains the original ticket information including purchase date, and is what is used internally when the original purchase date is needed.