vuejs - why "this.$refs.variable" gives undefined, but when I use the value of the "variable" it works

I'm a bit new with vue.js but still...I don't understand the behavior. Could someone explain why when I use a variable after "this.$refs.somevariable", I have an undefined result, but if I use the value of the variable, then it works ?:

<script>

export default {
    
  mounted() {
    //
  },

  methods: {

      scrollTo: function(anchor) {

        console.log(anchor); // "whoAreWe"
        console.log(this.$refs.anchor); // Undefined
        console.log(this.$refs.whoAreWe); //the DOM div "whoAreWe"
      
      }

    },

};
</script>

<template>
   ...
<button
    type = "button"
    @click = scrollTo('whoAreWe')>
    **Click!**
</button>  
...
<div ref="whoAreWe">
<div>
</template>

Try like following snippet: (this.$refs[anchor])

const app = Vue.createApp({
  methods: {
    scrollTo: function(anchor) {
      console.log(anchor); 
      console.log(this.$refs[anchor]); 
      console.log(this.$refs.whoAreWe); 
    }
  },
})
app.mount('#demo')

<script src="https://unpkg.com/vue@3/dist/vue.global.prod.js"></script>
<div id="demo">
  <button
    type = "button"
    @click = scrollTo('whoAreWe')>
    **Click!**
  </button> 
  <div ref="whoAreWe">
  <div>
</div>