Ok, the list looks like this:
A
B
C
D
E
F
G
H

Lets say I use your script starting with %bX set at 1, and set it to keep going until %bX <= 4

Delete line 1 and the list looks like this:
B
C
D
E
F
G
Delete line 2 and the list looks like this:
B
D
E
F
G
Delete line 3 and the list looks like this:
B
D
F
G
Delete line 4 and the list looks like this:
B
D
F

It isn't a bug..it's perfectly logical.