In addition to the scenario identified by Andrew Ferrier, where the cabin displayed is an artifact of the fare class and not the actual existence of those seats on the aircraft, there are several other scenarios where an economy fare can yield a premium seat, though admittedly I do not think any would apply to Avianca.
In the domestic U.S., various airlines deliberately publish[ed?] fares which book into a higher cabin. These fares were given economy fare classes, often in conjunction with "UP," giving rise to them being known as QUP, KUP, BUP, YUP fares (and so on); the fare codes might look like QUP8V or YRUPMZ. These usually carry restrictions that would not be applicable to a full F/P/A/J/C/D/etc. fare.
The idea is that an airline would prefer to fill a premium seat with someone paying a higher than average fare, rather than upgrading a frequent flyer who might be traveling on a cheap ticket. With the airline mega-mergers of the last decade and subsequent reductions in capacity, the airlines have less incentive to publish such fares, but they may still exist for some markets.
Automatic upgrades for full fare tickets
Some airlines will grant an automatic cabin upgrade for passengers traveling on full fares, regardless of the fare code. For example, United Airlines automatically upgrades all Premier members at the time of booking on certain domestic Y or B fares, regardless of tier considerations.
Automatic upgrades for frequent flyers
If you are an "elite" frequent flyer, it is possible that you were given the upgrades as part of your tier benefits. The Big 3 international U.S. carriers, for example, start to grant confirmed upgrades on domestic flights up to 120 hours before departure for their top-tier frequent flyers.