Do you know how Vapcell executes the DC IR measurement? There are a lot of ways to test it and a lot of factors could be of influence.
Below is a bit of an older topic, but as far as I can tell still relevant, but there is a lot of information (and interesting discussion) on how certain things could be of influence. (And therefore getting consistent and repeatable results.)
From this post onward is the most useful/interesting information: