It depends on the context that you use it in. Though it is a common military expression it is also used a lot in customer service when addressing a male customer, or to an older male to show respect. It is not used among friends or peers and generally not used among relatives, unless you are not close to them. My father asks people not to call him "sir" because he says that it makes him feel old. So, if you said "sir" to a friend, or to someone around the same age as you, his response may have been "wow" because he was not expecting it or you may have said it in the wrong situation socially. I hope that helps. :) One more thing... not really related.. when I was abroad in Asia I had sales people call me "sir" on accident. At first this surprised me because I am a female and the proper address would be "miss" or even "ma'am". I understand now that they were just trying to be polite but did not understand that "sir" was for males and cannot be used for females. In this situation a may have responded with a "wow."

the person might have been impressed with your eagerness. The term "Yes Sir" is usually a military response of a subordinate acknowledging to a superior that he/she has hard and understood a verbal order. For a civilian (non-military person) to say that to another civilian could surprise that person but could also indicate that you are very interested in pleasing them and your answer shows enthusiasm - hence the 'surprised' reaction - WOW!

If the person you were talking to was British they may have answered 'wow' because your answer was very polite.

because such expression is not used in ordinary colloqual language and people are surprised when you use it.

If I ask you do do something, please answer me in the same way.