I would check if other tier3 weapons are similar or not.
If not, then it would clearly point to being a bug. If yes, there is a chance it is actually intended, but I can't imagine why.
tldr; I would say a bug.
Int the XML of vanilla I see this:
passive_effect name="DegradationMax" operation="base_set" value="300,650" tier="1,6" tags="perkMachineGunner,perkBookAutoWeapons"/>
which would normally mean that quality6 machine gun would have 650 durability
but also this:
<passive_effect name="DegradationMax" operation="perc_add" value="-.2,.2" tags="perkMachineGunner"/>
which adds a +-20% random value to the weapon. Not sure why ALL the durabilities are the same in your case, maybe just chance. But generally durability should have a large random factor on any weapon, except that the weapons from creative menu all have the same.
EDIT: Sephiroth says even in creative menue the values are random. Possibly, such details can change between alphas.