Also, you're not going to be able to get all of the oil/butter out of the container. I thought I was finished with my bag of Shea but I decided to really scrape the heck out of the bag even though I didn't think there was much left and I got an extra 2 oz out of it (originally 3 lbs). For my liquid oils, I try to leave them upside down when a little bit is left for the next time I soap. I us usually get another 1-2 oz or so out of a 6 lb container (101 fluid oz)
ETA: I can't read. Just saw that you weigh the container as well. It's got to be sold to you in fluid oz then. The density of most oils is less than the density of water which is usually taken to be 1. That means if you have 8 fl oz of water, it should weigh about 8 oz too. But, since the density of oil is less than water, if you have 8 fl oz of oil, it will weigh less than 8 oz (lower density means lower mass amount for the space, volume, it takes up).