Some trainers just aren't as consistent (resistance at a given speed over a range of temps) as others. The 8 minute test is no fun (I just did it two days ago) but I'd rather do it again than monkey around with manually adjusting your FTP based on your perceived effort. If you're worried about 'throwing away' a session on your current plan just to re-test, consider that you may very well be throwing away multiple sessions until you get back to being dialed-in properly.