I have a column which has Data Validation set to a column of allowed items
(Article Codes).

Validation works right when one types entries in - only valid Article Codes
are allowed in the validated column.

However, when one enters values not one by one, but by spreading an existing
cell downwards in the validated column (drag from the little + on the corner
of
the cell), values are incremented and any resulting value is welcome by Excel
in the validated cell.

E.g., values allowed in the validation list:

HP9945
HP9980
HP9994

Entering values one by one will not allow HP9946.
However, if you spread HP9945 downwards, the cells under it
will accept HP9946, HP9947, etc.

Was this the way the feature meant to be?