Kaplan Exam Kit Question – Hemsworth Co

Question – You are the audit senior assigned to the audit of Hemsworth Co for the year ended 31 August 
20X5. You are ready to commence the audit of payables. The following procedures are listed in 
the audit plan: 
Procedure —  Selection method 
1 For 20 invoices listed in the payables ledger trace the amount recorded to the purchase invoice —  Start at a random point and test every $100th 
There is 2 questions which are contradicting , which are as follows:
Question 1)
Identify whether the selection methods described represent sampling. 
 Sampling  Not sampling 
1. (As per answer it is sampling and states that Procedure 1 describes monetary unit selection (MUS) which is a sampling method given in 
ISA 530 Audit Sampling., I agree that this is a sampling method which is systematic “Interval” selection.
Ques – 2) In respect of procedure 1, if the method stated to test every 100th item, which method 
would be described? 
? Monetary unit selection 
? Random selection 
? Systematic selection 
? Block selection 
Answer is Systematic selection
My question is in 1st why in answer it is then written that it is MUS ? because as per my understanding as well it is systematic selection only ?
Can you please explain?

Thanks in advance !