Are you only comparing data in a few of the online soap recipe calculators? If so, it's been my observation that calc designers typically borrow sap value data from other existing calcs rather than do all the independent research.
If you look at suppliers and manufacturers, you'll see a different story. For example, Soaper's Choice in the USA provides sap values that aren't necessarily the same as the values in the calcs. Reference Information | Soaper's Choice