He's basically using a hard calculation to determine the relative difficulty of the work instead of assigning RPEs. In so doing, he ensures the relatively difficulty of each set increases from week to week. Basically, he wants to make sure the average RPE increases from week to week, but he's using a more precise, harder mathematical calculation to make that happen while staying within the frame work of percentages.
4x6@70% = ~6 reps in reserve
4x6@72% = ~5 reps in reserve
4x5@76.5% = ~4 reps in reserve
Make sense?