The 'step' is required to some degree on all screw in choke systems. There's also a bevel there.
This is necessary for safety, to keep gas from getting behind the choke tube and blowing things up. It also keeps lead and wad material from shaving and accumulating at the junction and acting as an obstruction.
This results in some 'jug' effect and varies considerably by manufacturer.
In general, screw in chokes shoot tighter than marked.
That said, factory supplied Invector+ (flush length) have a poor reputation in constrictions of modified or tighter. The various extended aftermarket chokes for this system are a HUGE improvement. The 'Midas' chokes shoot way better than flush ones and are rumored to be made by Briley.
For skeet in my Invector+ guns I use Briley chokes marked 'Cylinder'. There is still some effective choke constriction in those chokes because of the 'jug' section that's just part of the design. They work very well for skeet.
