Whether checkout staff get paid the same as warehouse staff is irrelevant because they are different jobs.
Isn't this the crux of the case? - it's not 'checkout' staff but 'store' staff. Store staff are arguing that they are doing some of the same job as warehouse staff as they are lifting goods when stacking shelves. Their argument is that it is irrelevant that they also have to work on the checkout (although, if they had more sense, they 'could' argue that their job involves MORE skill as they ALSO have to work on checkouts AND deal with the public). Store staff are trying to argue that it is the same job as warehouse staff as there are certain duties which are similar.