If you were to run that same amount of slack but with no weight on the bike that will prematurely wear the chain.
This is a common cause for frequent chain slack. If the chain is adjusted for the proper amount of slack deflection with no weight on the rear suspension, then it will be way to tight in operation, and so the chain stretches from being over tightened, which occurs faster than the stretch that occurs from normal wear under load from the engine. Also, an over-tight chain will also pre-maturely wear the sprockets.
If you are curious, you could adjust the chain as you normally have been, ride for a while, check the slack noting how much it increased, then ride for a while longer, and note that the slack stops increasing, as the excess chain tension is relaxed. It's best to tension the chain correctly in the first place, but this would give you some peace of mind in knowing that the chain isn't constantly stretching, but rather just trying to correct the condition of being too tight.