write a component by yourself, which is passed to the subcomponent as number, but the result subcomponent receives stringtype

parent component:

<cmsProgress num=40></cmsProgress>

Sub-component:

<el-progress :text-inside="true" :stroke-width="20" :percentage="num" color="red"></el-progress>

export default {
    props:["num"],
    data () {
        return {
            
        }
    },
    mounted:function(){
        console.log(typeof(this.num));  //string
    }
}

what is the reason for this?