Changing ivs

well, to be honest, measuring IVs in percentile form doens't give you round numbers.

It's better to check if their IV stats keep being the same.

(I.E 41/45 for 91% IVs rated mons or 38/45 for 84% IVs rated mons)

This is not possible, so the most likely explanation is that you somehow checked wrong either the first or the second time. Where did you get their IV's both times?

Is it possibly when you checked the iv's of the pokemon, the calculator you used averaged the results of your iv combinations?

I know one of the popular calculators has given me results such as 85% before and that is not possible. Upon looking at it further, it was simply 84% and 86% being averaged out.

I believe you may have powered up the pokemon since the first iv check or maybe you even used a more reliable calculator to discover the true iv and thus the only 1% "change" in iv.

Edit... grammar, typo.

Not sure if it's just Chansey, but I had one that was 41
/45 pre CP change, then it became 42/45 after CP change...

I use Gamepress normally so pretty sure it was correctly measured the first time.

Sometimes an IV calculator will give you a range of possibilities for a pokemon's IVs, either because you didn't provide enough information or because there are multiple stat combinations that can yield the same CP & HP (I'm pulling this out of thin air, but an example might be a 14/10/10 mon vs. a 13/12/10 mon). The first time you checked you might have been looking at the minimum value of the range, while the second time you checked you might have looked at the maximum value. Alternately, if you checked the IVs, powered up the pokemon, and checked again the new level might have clarified the stats to a point where an IV calculator would give you only one possible answer, rather than a range.

What tool were you using to calculate your IVs? Did you round off a decimal point when you wrote it down? Did you use the same tool to recalculate IVs?

A change of 1/45 IV is 2.222%, so a change 1% sounds like a rounding error.