It’s possible that you got two different fares due to the airline’s pricing system. Airlines carefully manage their inventory to maximize the money they receive for each flight; and the way they do this is by selling seats at many different “booking classes” within each cabin. So, for example, within Economy class there might be “S” class, “W” class, and “Q” class tickets, each at their own price.
On each flight, the airline will sell a certain number of tickets in each class. For example, they might decide to sell 50 “S” class tickets at a lower price, 30 “W” class tickets at an intermediate price, and 20 “Q” class tickets at a high price. Once all the lowest-price tickets are gone, the price of the flight effectively goes up. The airline’s goal is to maximize their profits: sell as many tickets as they can at as high a price as possible, without setting the price so high that some of the seats go unsold. There are, no doubt, complicated algorithms that draw on past customer data, economic forecasts, special events happening in each city, and the phases of the Venus in order to assign the “right” number of seats to each class.
These booking classes are largely invisible to the casual customer, but the information should be on the itinerary somewhere. My guess is that the two tickets were sold under different fare classes; one of the tickets was the last in its class, and the next one to be sold was in a higher fare class.