How to find the trigger expression's ID stand for?

[Charles.Wang]

How to find the trigger expression's ID stand for?

I wanna to update the trigger expression automaticly by Nodejs json-rpc.
First step:Get the trigger info ->
client.call(
{
"jsonrpc": "2.0",
"method": "trigger.get",
"params": {
"output": "extend",
"hostids": 10084,
"search": {
"description": "Too many processes on {HOST.NAME}"
}
},
"auth": authid,
"id": 1
}
function (err, res) {
if (err) {
console.log(err)
}
else {
console.log("trigger info = "+JSON.stringify(res));
}
}
);

The feedback,the expression id was 13083 , "{13083}>300" means "{Zabbix serverroc.num[].avg(5m)}>300" ->
trigger info = {
"jsonrpc":"2.0","result":[{"triggerid":"13496","expression":"{13083}>300","d escription":"Too many processes on {HOST.NAME}","url":"","status":"0","value":"1","pr iority":"3","lastchange":"1487751422","comments":" ","error":"","templateid":"10190","type":"0","stat e":"0","flags":"0"}],"id":1}

Second step:Update the trigger expression->
client.call(
{
"jsonrpc": "2.0",
"method": "trigger.update",
"params": {
"triggerid": "13496",
// "status":0
"expression":"{Zabbix serverroc.num[].avg(5m)}>600"
},
"auth": authid,
"id": 1
} ,
function (err,res){
if(err){console.log(err);}
else{
console.log("update1"+JSON.stringify(res));
}
}
);
I got another expression ID at this time......... WHY???
trigger info ={"jsonrpc":"2.0","result":[{"triggerid":"13496","expression":"{13171}>600","d escription":"Too many processes on {HOST.NAME}","url":"","status":"0","value":"1","pr iority":"3","lastchange":"1487751972","comments":" ","error":"","templateid":"10190","type":"0","stat e":"0","flags":"0"}],"id":1}

I want to know how to find the expression id stand for? why they changed after updating?