The decrement counter shows the number of cans between the welding station and first light barrier of the eject station. If the value is wrong, the ejection of cans is out of sync with the faulty products.
The reason for a wrong decrement counter can be a reset during production. Reset the decrement counter only if the production is stopped and no cans are between the welding station and eject station! Never during production!
This is important to avoid that bad cans go to the downline.