The quote appears to illustrate the two types of "reasons" one could give when making a statement:
- The justification or evidence that the speaker has for making the statement.
- A reason that the stated phenomenon has occurred.
Thus, in the first example, the speaker explains what evidence led her to conclude that the invitation was forgotten:
(1) You must have forgotten to invite them [statement], because they didn't turn up [justification or evidence for the stated conclusion].
While in the second example, the speaker is explaining why the phenomenon that she is describing (blushing, in this case) has occurred:
(2) He blushed [statement], for he knew he had been caught out [reason for the blushing].
The following would be examples of the other type of reason for each of these statements:
(1) You must have forgotten to invite them [statement] because you have been so busy at work [reason for the forgetfulness].
(2) He blushed [statement], for I could feel the warmth in his cheeks [justification or evidence for my stated conclusion].
Because and for can each be used for either of these distinct types of reasons.