The gas oven temperature mistery

I live in Argentina, where normal cheap ovens are gas ovens without a thermostat. A knob allows me to adjust the size of the flame and that's it. I know this is almost prehistoric by first world standards, but it's normal here.

Since I often pull recipes from the net or foreign cook books (Betty Crocker's and such) that give oven temperatures, I bought an oven thermometer.

My experiences with the thermometer were not good...

Let's say the recipe called for a temperature of 180 C (Moderate heat). I had to set the oven to the lowest setting for the temperate to stay around this number (according to the thermometer). At first I buttumed my oven had a very high gas output, and the lowest setting was actually moderate... but cooking took more than twice the time in the book.

When I repeated the recipe, I then set the flame to the middle setting. It came out perfect, and in the exact time listed... throught the cooking the thermometer marked between 210 C and 230 C.

This lead me to believe my thermometer was off... so some months later I bought a new one (don't know about the first one, which is a different brand, but this one is German made... both look the same as oven thermometers sold in Amazon and other on-line vendors).

The experience with the new thermometer was the same... so I did a bit of testing: I put both thermometers in the oven, heated it, and compared: they both marked the same temperature (with no more than a 5-10 degree variation at some times). I moved one of the thermometers around the oven, testing different locations (standing in the middle, hanging from the front, etc.) and they both kept marking the same temperature. So know I think my thermometers are working just fine.

Does anybody know what's going on here? Are gas oven temperatures somehow different than electric oven temperatures? Should I be adjusting by some factor?