Sorry! This is a strange one!
It appears that the 2015 and 2016 battery are different capacities, where as it appears that the 2016 and 2017 batteries are identical capacities. However, it seems quite possible that the batteries in the 2016 and 2017 are different, considering the top case itself is different, and minor tweaks to battery electrolyte recipes for minor performance/stability/longevity improvements are
extremely common. From what I can tell, it seems impossible that the battery is indeed from 2015.
If so, that leaves three possibilities that I can personally think of:
1 - the Apps are reporting something incorrectly (as you have used two different programs, this seems less likely unless there is a compatibility issue with High Sierra for some very strange reason)
2 - when the battery was given a birthdate, there was an entry error (if the born-on month/day of the battery is only several (say 2-14 days) BEFORE the born-on month/day of the computer, this to me seems more likely - such errors do happen)
3 - there is a problem with the battery causing it to incorrectly report (I think this seems less likely than the other two if you are getting proper performance)
If the battery is from 2015, we would expect it to likely have a somewhat lower actual capacity today than its original design capacity.
For reference, my 2017 rMB was 'born' 6/26 and the battery was 'born' 6/17, according to third party battery Apps. I'm running Sierra.
I am guessing you have already tried resetting the SMC?
