Different but similar Xert HIE vs W prime

I know there are some people on here with a depth of knowledge on these two parameters. When I discuss this on the Xert forum it is a case of supporting their approach and it may be all very good but they are not agnostic so that is why I ask here. When I did an effort yesterday and look at it with intervals.icu I have a set W’ of around 8100 kJ. when I look at my Xert signature the HIE is 16.7 nearly double that number. So in an effort yesterday the W’ went to zero, but of course the MPA (which is effected by HIE) only dropped a bit, now the interesting thing, if I edit my signature to have an HIE of 8100 the MPA drop is almost identical to W’… What have others found if they compare their W’ and HIE for similar effects? just curious. I realize they are not identical but this experiment makes me wonder.