A couple issues right off --

If you're working with views, CALCULATE works off the latest version of the data, SQL-SELECT works off the last requeried version (this is from behavior, not knowledge of internals so may not be fully accurate). So, if you're editing an existing record, you may be seeing the difference between edited and original versions of data. The behavior may be the same for tables but I haven't tested this.

You may also see a difference in the handling of null and empty values between the 2.