Hi Stu4084,
Blended Talalay latex is normally made with a relatively small variance in ILD either in the “average” of the 9 measurements they use to rate the core or in the variance across the surface of the material (unlike Dunlop or 100% natural Talalay which can have a wider variance). There is more about this in post #6 here. It would be very uncommon that the small variations between two different cores is something that people would feel in a split layer mattress where the ILD is the same on both sides.
If the difference is that noticeable it’s quite possible that “human error” may be the cause and that the difference is because they actually gave you the correct “split” firmness even though it was listed on the order as being the same.
When you receive the replacement if it seems to be the same as your current softer side then that’s what may have happened.
Phoenix