A DNA land and GenePlaza DNA test reports that I am 73% South Asian, 24% Central Asian and 2.8% Mediterranean Islander. It then concludes that I'm mostly West Eurasian.
How is this possible when I have no European ancestry?
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The ancestral calculators use reference populations for their categories and predictions. Then then use the markers they find in your DNA to match you up as best they can.
Some testing services do this better than others. And some are good with certain populations but not so great with others. Though there is some overlap, they draw on different sets of reference populations and/or create their own. Some consider anyone who has all 4 grandparents from the same place and known ethnic group to be good enough for a reference (it's generally not).
You may match up well with people from the West Eurasian reference population they used. Or they may be guessing based on proximity. Ancestry calculators often literally map out the location of the ancestry categories and then give a prediction based on where you'd be on the map after being pulled in various directions (weighted by the percentages).
Your south and central Asian is being pulled slightly west by the Mediterranean Islander. This gives you west Asia. Note, it's not saying you're European. "West Eurasian" is where Europe and Asia overlap or meet, plus places to either side.
The other (usually better) way to determine ancestry is to assume potential admixture (mixed heritage) and postulate multiple points of origin (such as with Oracle). For example, you might get 25% eastern Mediterranean, 50% Indian, and 25% Afghani (I'm making this example up).
My recommendation is you upload your raw data to other services and see what they say. Gedmatch is the most comprehensive and gives you a wide variety of calculators to choose from. Also try FTDNA and MyHeritage. (23andme and AncestryDNA are good companies but do not accept uploads of raw DNA.)
If you didn't test with a company that gives you raw DNA results that you can upload, consider testing with one that does. We just had a lot of Christmas and New Year's sales, and some might still be going on.
An explanation of various Gedmatch tools is here: Finally! A Gedmatch Admixture Guide!
Here is a guide to the underlying ancestral groups that make up (on average) various modern-day populations: Your Regional Ancestry: Reference Populations