This is a classic example of failing to see the ambiguity in a polling question.
Those GOP voters who want to see witnesses may still believe the process is an absolute joke, and are not be thinking of anti-Trump witnesses at all, but rather just want to see the Bidens, the whistleblower, etc., on the stand under oath. Not because they think there's any merit to impeachment, but because they think it is a sham that should be exposed.