It's to do with the way in which the variable t is defined. Since you'll be writing your own answers in the exam, you can choose the way in which you define t.
You could choose:
- t to be the length of time from the outset of the project (as has been done in this second question you refer to). Since the DPP is the time until profit measured from the outset of the project, then in this case t gives you the DPP (or you round up to the next cashflow after t if income is discrete).
- t to be the length of time from the start of the receipt of income (as in the original question you refer to). Since the DPP is the time until profit measured from the outset of the project, then in this case you'd have to add on the time period from outset until the income starts to find the DPP (and round up to the next cashflow if income is discrete).
In all cases, the DPP is the time until profit measured from the outset of the project. What variables you define in a question to work this out is up to you. The important thing is that you know what you've defined, so you can calculate the DPP properly.