I’d also suggest that climate can have an influence on degradation — even just seasonal variance some owners see in say depths of winter vs summer where some of that range comes back each year, vs what some early EV owners saw as true hits on their battery life because of vehicles that lived in very hot or cold environments for months/years at a time. Then of course, there is real L-ion battery degradation over time — they all do it, some more than others, and there is generally a larger drop-off in the early months of battery life, than how it tapers off to a more consistent loss over time from there. I doubt this is your problem or something we as Volvo PHEV owners can effect, but it’s also known with L-ion tech that keeping a battery at max charge or close too it, for too long is bad for it’s longer term max charge expectancy — definitely a reason why e.g. with a Tesla, you never want to consistently charge to it’s (closer to real) 100% level and let it sit there “unused” for long, and even Apple in their last iOS release put in a whole new set of optional battery charging logic trying to not charge above 80% until closer than you really need it to increase longevity…
One immediate thought is, doesn’t your 2017 have a 9.2kWh battery, whereas a 2018 has 10.4kWh? I think so, and I could be wrong on this additional point, but didn’t physical external size of the battery within both cars remain the same as that capacity increased — meaning Volvo is doing something inside changing cell configuration and/or formation to provide additional capacity, just as they did again with 2020 models like mine with the bump to 11.6kWh? Given that, how Volvo has to create estimates from voltages/amperage/etc., their algorithms for capacity and range will be slightly different, so how good those estimates are could also vary, AND problems/life-expectency/etc may well be different over the longer term because of what “the battery” is really made up of inside. I say those last couple things as a former Tesla owner who had one of the first 90kWh batteries off the line… long story short, but mine was ultimately found defective and replaced because of faster-than-expected degradation, and while I’ll never know for sure, interesting thing is it looked the same on the outside and everything I could tell from my owner’s POV, but the replacement physically had a different part number than the original did… At least one other owner I connected with on another forum with the exact same battery, built a week earlier than mine, had similar problems …but owners that took delivery of the same battery a few months later didn’t see any more extreme problems like ours and had that new part number. Anyway, different mfgr examples, but battery sizes and perhaps different internal tech may be part of why you are seeing relative degradation differences between the two Volvo you own, even when accounting for different kWh sizes.
I’ve thrown out lots of non-specific supposition, and will leave it all at that. I’m not trying to create some conspiracy theory , just offer food for thought as I think you may have been after. Hopefully other 2017 XC90 T8 owners will chime in with their reality, you can compare against. Good luck.