You have to think back to the days of paper tickets, and what you would do if you had a lot of sectors, but then you changed a few (or had them changed by AA). The answer is that additional coupon booklets get stapled behind the first ticket, with sequential numbering for the following booklets and perhaps an out of sync number for changes. Each coupon could have between 2 and 4 sectors on it (some airlines internally had 6) and anything more complicated than NCL-LHR-AMS would need a second coupon. This explains lots of things, including the one year expiry on tickets: the date here is redacted (lets say it was 1 January 2002), but it's in one place, each line has a reduced date format, which only makes sense in relation to the big date box. If one sector was 2 years later in 2004 there would be no way to indicate that. Note the fare.