I think it's documented in mskb#Q185258.

>Hi Everybody,
>I was wondering if anybody has seen the following when using the spinner control:
>
>The control does not display the value correctly under the following options:
>Increment Property to any value non whole number, Exp 0.10, 1.5
>InputMask Property set to display the non whole numbers, Exp 999.99
>
>When using only the the spinner buttons, it seems to round the number being displayed.
>
>If you enter in a value into the control, and then use the spinner buttons it seems to work properly
>
>Thanks in Advance,
>Joseph Schaff