Hi gaeila,
Not to belabor the point but there are only two possibilities. Either the weight you measured wasn’t correct or the topper wasn’t a 5 lb topper. It isn’t a matter of “luck”.
If a material is the same type and density then from that perspective it wouldn’t matter in which case I would make a choice based on all the other objective, subjective, and intangible parts of my personal value equation that were most important to me (such as price, return policies, the knowledge and guidance of each one when I talked with them etc).
Phoenix