Why do ESL speakers sometimes use “sir” to women?

Question

When I use a website with a large numbers of users for whom English is not their first language (specifically, fiverr.com), I often get addressed as "Sir", even though I am a woman. How come?

My hypotheses are:

1. Learners of English in some countries are being explicitly taught that "Sir" is appropriate for individuals of any sex (kind of like default masculine pronouns).
2. Honorifics in other languages are gender neutral, so they assume the same is true of English.
3. They are simply incorrectly assuming I am male.

This is different from the question https://english.stackexchange.com/questions/121727/can-sir-be-used-to-address-female-officers, which refers to the military. I am a civilian.

Answer 1

Perhaps your readers interpret your signature, espertus, as masculine because of the "-us" suffix, which is masculine in Latin.