Part of the reason for why they might not sound the same are unit to unit variation invalidating the accuracy of your EQ's - ie you can't be sure that each IEM you own is EQ'd to the exact same curve - so in that instance you can still explain your perceived sound differences due to differing frequency responses. The second one is the IEM's may not fit in your ear in the same ways thereby creating some different interactions that again change the frequency response you perceive - although I'd think this phenomenon is less than the same phenomenon with over ear headphones. The third reason is that you might not have level matched your IEM's between swapping them to compare - louder sounds better / also affects tonality. Fourth reason is that it's just hard to compare headphones/speakers/IEM's - it takes time to swap them over at which point you've forgotton some of the finer nuances of how the previous one sounded - as detailed remembering of how something sounds only really lasts a few seconds.......that doesn't mean you shouldn't try to compare your IEM's headphones/etc, just that it's not easy.I know they don't. I have different IEMs from Shure, Moondrop, Samsung, JVC, Massdrop, Pinnacle and others. Also headphones from Sennheiser and HiFiman and Koss. They can all be eq'd to offer the same frequency response very easily these days on PC or mobile. They absolutely do not sound the same even when EQ matched. Similar(ish), yes, but a long way from identical. All these publicly available correction curves like at AutoEQ are great for correcting gross errors but in fact you can't make them all sound the same. It's also worth remembering that almost all IEM measurements over 8000Hz are known to be unreliable, so using them in a parametric EQ correction doesn't make much sense. And anyway most EQ corrections require really significant precut, -7 and -8 dB is not unusual; how many phones can really lose even 3 or 4 dB and still have great sound? Very few.
However, if you take two identical models of headphones and you actually measure those units and EQ them to same curve throughout the frequency range then they will sound the same - I know that because I've got more than one unit of the same headphone, measured them all on my miniDSP EARS and did an experiment once to EQ a few of those units to the exact same curve - they did sound the same, but that's because you're completely removing the unit to unit variation and it's the same model of headphone - so frequency response doesn't lie - just you don't always know what that is.