I was killing wither skeletons with Smite V and Strength II diamond sword. I saw that I had 13 kills. I killed a wither skeleton and checked my stats: still 13. Then I killed another: now 15.

To reproduce:

1. Spawn in a bunch of wither skeletons.
2. Get a Smite V diamond sword (netherite probably works, too)
3. Apply Strength II
4. Kill the wither skeletons and count how many you have killed
5. Check your stats.
6. Repeat 4 and 5 until you notice a discrepancy.