Right or wrong, "refurbished" always just seemed like a fancier way to say "used" to me, and I never buy anything used so I always ignored them.
I noticed in Canada that the regular prices of the "Pro" lenses increased last year, and other than occasional sales and some differences between retailers, I haven't noticed any fluctuations.
In theory . . .
Used could be used for years.
Refurb is formerly demo gear or item that has been returned (for whatever reason). So used very little or not at all.
Used would have had no one other than the seller doing a QA on it. And the seller could be trying to pass a problem on to you.
Refurb, IF done right, has a QA check on the gear, and any issues fixed. But as I've said, there could also be NO QA done.