If you've been on holiday or off sick your Sunday Premiums drop and are included in the relevant payment. Have you been off at all? Done a shift swap?
Contracted hours which fall under night premium and sunday premium automatically make up the calculation of the weekly pay, holiday pay is paid at either the contracted rate (so at least what their weekly pay is which includes the sunday premium and night premium they do as part of their contracted hours) or the average weekly pay based on the last 12 weeks of pay data, so if the OP has done any overtime without going off sick in the 12 weeks before they take their holiday, their holiday pay should be higher than their contracted.
Edit, I've just realised you may have meant the itemised sunday premium element of their payslip, yes, they wouldn't have actually lost any money, but it would appear less than normal.