That's what happens when you read 400-year-old English through 21st-century eyes. You assume that the meanings of words haven't changed over time, when in fact many have changed dramatically.Clearly, "should" is less certain than "shall", by definition.
I "should" be able to fix my car on Sunday...iffy.
I "shall" be able to fix my car on Sunday...a more absolute statement.
More confidence in the latter statement.