Yep, it's stability:
While I'm not really expecting anything of note to come out of this, there are some rather unusual modules updated this time. Of particular note is the DSi-mode firmware, which doesn't seem to have meaningfully changed since 2013 (outside of New 3DSes getting their own version because the hardware is different).
Old 3DS report
New 3DS report
https://en-americas-support.nintendo.com/app/answers/detail/a_id/231/p/430 said:Further improvements to overall system stability and other minor adjustments have been made to enhance the user experience
While I'm not really expecting anything of note to come out of this, there are some rather unusual modules updated this time. Of particular note is the DSi-mode firmware, which doesn't seem to have meaningfully changed since 2013 (outside of New 3DSes getting their own version because the hardware is different).
Old 3DS report
New 3DS report