Yes it is inherent of all black tanks. The outlet is typically on the side of the tank not the bottom. The outlet is also not flush with the bottom of the tank.
There will always be fluid left in the tank. The drain is not low enough to get it all out. Add in that the tank/RV are not always level. The tank maybe tilted the opposite way of the drain.
In the end it is not a problem. There will always be residual water in the tank.
You really don't want tank totally dry anyway. It is a good idea to dump a 5 gallon buck into the tank after dumping.
The tank does not need to be spotless,you are just going to put stuff in it again....and again...and again...anyway.
The tank is designed to hold stuff no need to be anal about it being spotless.