I bought an economy ticket and was issued first class ticket

Congratulations, I wouldn't complain! Airlines are pretty good these days at managing their passenger load, but sometimes they might overbook a flight or make some mistake and move people around between classes.

Worst case, they made a mistake (and notice) and you'll fly economy just like you booked. Best case, you'll enjoy bigger seats and fancier food.

It is perfectly possible that there is no Business/First class cabin on the plane - you don't say how long the flight is. For example, British Airways flights within the UK are sometimes booked as "C" class tickets (Business), even though there is no business class - it simply means you have an expensive, flexible ticket with benefits (e.g. lounge entry). That might show up on some websites as Business class, even if it really isn't once you actually get on to the plane.

So don't get your hopes up too much!

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.

-UP Fares

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.

I fly regularly with the same few airlines for business (even though I always purchase standard tickets) and have noticed that maybe one in every 8-10 flights, I am upgraded in some way, whether its 'first class' on a short haul flight (larger seats) or invited to the first class lounge on longer flights without much notice. They don't directly tell me this information, I notice when I receive the boarding cards, or check my account, that I have been upgraded free of charge.

I always presumed that this was loyalty in some way. I appreciate it, and don't ever question it. It makes me think highly of them and likely to recommend them to colleagues about the positive experiences that I receive when flying with them.

Further Information: I believe this is more likely to happen if flying solo than in a pair or a family. In my experience it has only happened when flying on my own.

Recently I was shopping on Expedia for a KUL<>LBU ticket with a hotel reservation together. The total price was a bit high, but not that much. On the days that worked for me, the return flight was in Business class, even though I asked for economy. Turns out economy was full, and Expedia just fulfilled my request by selecting a seat in the next available class. It was clearly indicated, but I could have missed it had I clicked clicked clicked through the options and just purchased the package (which I did anyway as I needed to go).

Maybe that's what happened during your booking.