[Bug Report] don't understand the logic of el-select

### Element UI version
2.13.2

### OS/Browsers version
Google Chrome Version 85.0.4183.102 (Official Build) (64-bit)

### Vue version
2.6.12

### Reproduction Link
https://codepen.io/matamune94/pen/VwaGWJO?editors=1010

### Steps to reproduce
create vue and add el-select

### What is Expected?
box select show Option 4 not Option 5

### What is actually happening?
v-model="value1" have item is { _id: 'Option4', name: 'Option4' } but box select show Option 5