The people responsible for the Lancet published research weren't the same as the people responsible for the earlier estimates.

The figure in the report of the Lancet piece for pre-invasion infant deaths is a lot lower than the figure in the UNICEF based site for of children under five (though it's not clear if "children under five" means the same as "infants").

Presumably the figures used by the Lancet is based on some other source, which would be identified in the actual research report.

The apparent discrepency does not in itself invalidate the new report. It coulds be that its figures here are innacurate, or it could be that the earlier figures were inaccurate. Moreover from the press sumary it looks as though the main focus of the actual research was on trying to get figures for violent deaths of people of all ages.