You did not get the desired list of Purchase order using Webservice though you got the same using Respective report.
SAP Business ByDesign
Because of specific 'timeZoneCode' value( ike UTC) passed via the query parameter related to 'Ordered Date Time'. so only those Purchase orders created with that time zone are appearing.
Do not use the Time zone in the query.
If your requirement is to fetch the POs of current year, you can use the 'IntervalBoundaryTypeCode' as '9'( Greater Than or Equal to) and 'LowerBoundaryOrderedDate' as 1st January with no specific time zone mentioned.
As suggested below,
<LowerBoundaryOrderedDate timeZoneCode="?">2018-01-01T00:00:00Z </LowerBoundaryOrderedDate>
QueryPurchaseOrderQueryIn, Purchase ordes missing, Webservice query not working , KBA , AP-POP-PO , Purchase Order , Problem