LED burn-in is the slight change of color that occurs after a few weeks of operation. It's most noticeable in the white bulbs and sets because pigment in the LED lenses compensate for color change.
What this means is that if you have two identical sets of lights and you plug one in for several weeks then plug in the second set, they will be a slightly different color. The difference won't be dramatic and if you didn't tell an onlooker that there was a color difference, they might not even notice.
So this means that even if you purchase extra lights this season, they may not all be exactly the same next year.
Keep in mind as well when purchasing lights from year to year that LED light colors may vary slightly from year to year. And the color may vary slightly between manufacturers/distributors. (So if you purchase 2 sets from one vendor and 2 sets with the same item number from another source, they may not be exact.)
Incandescent lights don't have this issue with burn-in. It's a side effect of the aging of the semiconductors that emit light in the LED lenses.
Well, I hope that answer helps you understand a puzzling phenomena.