The issue you're hitting is due to the country you're purchasing the ticket in.
Airlines frequently list difference fares based on the "Point of Sale". eg, if you buy a ticket from a US "point of sale" it might cost more than buying the exact same ticket from a UK "point of sale".
By default, ITA Matrix uses the departure location as the point of sale. So when you're asking for a ticket from MDE to CTG, it's presuming that you will be buying the ticket in MDE (or more correctly, somewhere in Colombia). That's why when you search for that flight in ITA, it shows the Colombian Peso. There is an option to change the currency it's displayed in, but this simply does an on-the-fly conversion from one rate to the other.
If you look down the bottom of the ITA search screen you'll see an option for "Sales city". If you set this to an airport in the same country as where you will be buying the ticket (eg, SFO if you're in the US) then you'll see that the currency changes to your local currently - but also the prices change as now it's going to show the price of tickets available for sale in that country. In the case of your flight, at the moment the cheapest price when the ticket is bought in Columbia is US$36, but when bought in the US it's US$91.
If you want to get the lower price there's only one option - buy the ticket in a country that it's available. In this case, if you go to avianca.com and set your country to "Columbia" it will quote you to cheaper fare! Online travel agencies based in Columbia may also be able to give you this fare.
However even this will not always work. Often travel websites will only allow purchases using credit cards from the country of the website, so although you can see the cheaper price on Avianca.com, you might not be able to actually purchase it.
In most cases you can actually see what I've described above in the "fare rules" for the fare. When looking at the final summary screen on ITA Matrix you'll see a link to the "Fare Rules" for the fare. If you click on that you'll get all of the details of the fare, including in this case :
TICKETS MUST BE ISSUED ON AV OR TA AND MAY ONLY BE SOLD IN COLOMBIA
OR - TICKETS MUST BE ISSUED ON AV OR T0 AND MAY ONLY BE SOLD IN COLOMBIA
If you set the point of sale to somewhere elsewhere in the world, you'll now see that the fare rules include :
TICKETS MAY NOT BE SOLD IN COLOMBIA/VENEZUELA.