Different Quantity in Lists

I have a number of lists, most are OK, but several of them have a different quantity count in “List” mode (pic 1) and a different quantity mentioned when you actually go in to the list (pic2).
Any ideas why?

Pic 1

Pic 2

Other different counts are:
17 - 18
23 - 31 (as mentioned)
2406 - 2402
3235 - 3234
2094 - 2091
82 - 84
4 - 3
949 - 950
50 - 52
50 - 61
50 - 93
5 - 6