For anybody wondering: The weird amount of time is because with a commercial CA it needs to be possible to "carry" some validity during renewal. If I need a $10 Doodad and they're valid for exactly one calendar year, if I renew the Doodad on Monday instead of the following Sunday because I know I'll forget at the weekend, I am losing almost 20¢ of value. People get disproportionately passionate about stuff like this. So, the CAs credited your remaining time on the previous certificate - if you renewed with them, if you had six weeks to go but renewed a 3 year cert early you'd get issued a 3 years + 6 weeks cert.

As the maximum expiry shrank (to improve agility and encourage automation) the slack for granting such extra periods shrank too, withh "3 years" it was actually 39 months, maybe a bit more depending on how you squint, now it's exactly 398 days because Apple said so.