This mistake is probably caused by the vague memory of a simliar 3-4 joseki.
So what's different here?
White may play in the first diagram expecting Black to make a two-point extension here; as in a typical continuation of the 3-4 point joseki.
However, Black probably wouldn't do that, because the two-point extension on the fourth line doesn't have the same meaning as it would in the 3-4 point joseki - it doesn't secure territory. Black may instead choose to seal white in with a.