SORDER.SHIDAT_0 & SDELIVERYD.SHIDAT_0 not matching

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Hi,

This may not be an error as much as a question as the internal workings of Sage X3. I've been setting up reports for our Sales department, but they weren't lining up wtih our Actuals. I realized that I was utilizing SORDER.SHIDAT_0 instead of SDELIVERYD.SHIDAT_0. Using the latter gave us matching results. 

However, my question is why would these values not be the same? They're both listed as Shipment Date in the Table Dictionary and I've linked them correctly. Is this normal? If so, why does it do that? 

Thank you in advance for your help! (v7patch8)