It’s because you’ve assumed the “convert” was in NPP, but really it’s in WordPress.
When you copy a selection from Excel and paste into a textual context, every line ends with a newline character (so if you only select one column, every CELL ends with a newline character), and multiple columns are separated by TABs. When you paste that text into WordPress, it appears to be converting the newline into a <br />. But when you paste it into NPP, it just interprets it as an end-of-line.
I don’t have WordPress convenient right now, but my next debugging steps would be:
What happens if you paste into NPP, then re-copy from NPP and paste into WordPress?
What happens if you paste into NPP, then use column-select (alt+drag selection), copy, then paste into WordPress?