Also, the initial martian colonists most likely went through a number of health checks and acclimation tests.
People from high altitude environments would have done better in those tests, resulting in statistical anomaly of some importance. As a result, the martian colonists as a population were not average Earthers, but had somewhat higher percentage of people from high altitudes. Then there's the amount of initial colonists who simply couldn't or wouldn't adapt to the conditions, and again this percentage would be higher within the "lowlander" parts of the population.
And there have already been several generations of Mars-borne people, which means the initial higher percentage of "highlander" genes (and the first one to utter the pun will be cleft in twain with a claymore) have spread to the population. Add the time when Mars was socially and economically almost isolated from Earth and other parts of GTA. The population of Mars stayed static during that time, but that doesn't mean no one left it. Those who felt it was too harsh left, those who could and wanted to stay, did so.
Note that this doesn't mean everyone on Mars was a space sherpa, but a statistically significant part of population had some of the traits associated to high altitude populations. One of the sources I used here.
After the collapse of Delta Serpentis node, the new colonists are more representative of the entire human population of Earth. However, simply due to practical reasons the portion of colonists who have genetic heritage from Andes or Tibetan areas is slightly over-represented.
Please don't mistake this as eugenics of any sort or even selective breeding, it's simply what I would expect to happen quite naturally. Also, it's still not all of Martian colonists that have these traits, more like 10-20% at the most - and that's of the population before the Great War and subsequent influx of new immigrants. There are no reliable statistics of the genetic diversity of Martian demographics available at the moment.