They've explained for several quarters now
That they are allocating parts that are in shortage to iPhone production over iPads and Macs. That makes sense since iPhone is their most important product at nearly 50% of revenue, and is also important for Services income as increases the installed base of iPhones helps future Services income.
Even buyers who are already iPhone owners figure into this, because the second and third owners of iPhones (via hand-me-downs in a family, trade-ins, or eBay sales etc.) becoming first time iPhone owners are the ones mostly responsible for expanding that installed base.