The way the alert is currently configured it is triggered if the node name is LA, NY, MX OR average response time is greater than 75. So you should be getting alerts for LA, NY and MX no matter what the average respone time is. You should also get alerts for any other node that has an average response time greater than 75. Try configuring your alert triggers as shown in the screen shot below.
Also, you are getting 71msms because the alert does some autoformatting for you. It automatically adds ms when you use the $(Node.AVGResponseTime) variable. So if you remove the ms after the variable in your alert message then it should come out as 71ms.