Thanks for correcting my error.
If that’s the case, I really wonder why Canon opted to make the RF-S 18-45mm, instead of the ‘classic’ 18-55mm APS-C kit zoom or a 15-45mm design like the updated M kit lens (which IMO is a more useful walkaround range on APS-C)?
The answer that comes to mind is to encourage crop R kit lens owners to buy more lenses to expand that limited focal range. That sounds Canon-like…
The EF-M 15-45mm does benefit significantly from the closer registration distance compared to the EF-S mount 18-55mm lenses. With the rear elements closer to the sensor, it allows them to be significantly larger than the rear elements of the EF-S 18-55mm lenses. This allows more light to fall on the edge of the sensor at straighter angles. At least in theory you get better edge resolution and less light falloff compared to an EF-S zoom that is 15mm on the wide end.
Only Canon knows for sure why they made the decision regarding the RF-S kit lens' focal length range. It could be more of a marketing department decision, or there may be a compelling technical reason that affects either performance or manufacturing cost and thus the ratio between performance and cost. Based on past observations, it seems Canon tends to begin the design process by deciding how much a lens or body will cost to manufacture and then designing something with acceptable performance to meet that goal. What they consider acceptable, both in terms of price and performance, seems to shift based on competitive conditions in the marketplace.
Maybe they decided an RF-S 15-45mm would be too expensive to be competitive in the market space a future lower end entry level APS-C body might be aimed at? One of the biggest knocks many had against Canon's "budget" EOS RP was that the original kit offered which included the RF 24-105/4 when the
RP was announced in 2019 was almost double the price of the body only ($2,399 vs $1,299). Even with current pricing and paired with the non-L variable aperture STM lens introduced a year later in 2020, the RP with RF 24-105mm f/4-7.1 STM lens kit is 130% the cost of the body only option ($1,299 vs $999 = +$300). Compare that to contemporary Rebels. The 18-55mm kit was typically only $150 more than the body only. The Rebel T8i/850D announced in early 2020 was $749 for the body and $899 for the single 18-55mm lens kit. (All prices in USD from authorized dealers in the U.S.) Sure, the 24-105/4-7.1 is a FF lens and the 18-55mm isn't, but when the FF body doesn't cost that much more than a Rebel (and
less than an 80D or 90D at the time), all some folks can see is price. I can remember not that long ago when temporary "instant rebates" would put the price of a Rebel kit with 18-55mm below the body only option. (Yes, those days are
probably gone forever.)
But a difference between 18mm and 20mm registration distance isn't that significant when the sensor/image circle is the same size. The real difference between the requirements for an RF vs. EF-M lens design is the 61% larger required diameter of the image circle, not a 2mm difference in flange distance. As long as the rear element is smaller than the throat diameter restriction of the larger RF-S mount, they could let the rear element protrude 2mm further and use the same design for an EF-M and RF-S design.
The need to accommodate sensor movement requiring a larger image circle for APS-C RF bodies with IBIS, such as the R7, might also have come into play.