[Bug Report] Table span-method applies span to incorrect cells

### Element UI version
2.15.7

### OS/Browsers version
Windows 11, Chrome latest, Edge latest

### Vue version
2.6.14

### Reproduction Link
https://jsfiddle.net/mztgcskq/23/

### Steps to reproduce
Set span-method property for the el-table component, apply some spans for columns

As seen in the fiddle console output span is applied to column index 4 of 6 cells, but it is actually displayed at column 7. If previous span (for column 0) is not applied then it works as expected

It is also clear for the second table which uses completely hardcoded intervals (in case it might look that dynamic calculation of column spans in the first method is not correct)

### What is Expected?
It is expected that returned column span is applied to correct column

### What is actually happening?
Column span is applied to wrong column (looks like shifted by previous span value in a row)