help

So I have a question- I haven't been with my ex since April but the last time we spoke was November. The last time we spoke was civil but he told me that whenever I see him to act as though I don't know who he is and basically keep it moving. Since that conversation I have seen him numerous times all of which he doesnt know that I have actually noticed him i just always end up looking as if i didn't all of these times he has looked the other way or acted as though he is doing something so he never has made eye contact. My question is "why"? If you are done and you have moved on why go out of your way to not look at me? I know he doesnt know that I have seen him before he actually walks away or looks the other way but if you have moved on why can't you make eye contact and act like I don't exist because he had no problems actung like i didnt exist when we were together