[asveikau]

I believe that one had [ʃ] (English 'sh') when the Spanish got there but then there was a change in Spain (in Castilian only even, not in Catalan or Galician) from /ʃ/ and /ʒ/ -> /x/ (like the 'ch' in loch) and they brought it to the Americas.

Somehow those Spaniards brought that change to /x/ to all those Nahuatl place names but they never brought /θ/ to everyday speech in Mexico. I scratch my head at that.