if use mouse wheel to scroll, the speed is slower?

as attached, if I drag the scrollbar in the upper right, the flow is very fluency, but if I use the mouse wheel to scroll, the speed is slower and always has some lags (that is, the scrolling is sometimes slower and sometimes sudden faster, and sometimes not scroll even scroll the mouse wheel)
what is the problem come from? or it is default behavior of gwt celltable and scrollbar( the upper part is a scrollpanel enclose a celltable)?

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