I've found what I guess is just a display error that happens with certain weapons going from dual wielding to single wielding doing the following.

1. Say I have my Vampric Axe of starlight equipped, the DPS shows as 11.09. (I was confused at first since it only says "Damage" untill mousing over said it's the actual DPS).
2. I equip a grey axe 2-2 damage in the offhand and the DPS goes down to 7.27 as expected since it's a much weaker weapon but you still have to spend the time swinging it.
3. I unequip the Vampric Axe and the DPS drops to 2.83 since it's just the grey axe equipped.
4. I equip the Vampric Axe again and it goes back up to 7.27 as you'd expect.
5. I now unequip the grey axe and the DPS goes back up to 11.09 since it is just the Vampric Axe equipped.

That's all fine but the problem i've seen is some weapons I equip with the Vampric Axe if I go through that process of having both equipped then unequipping the Vampric Axe, equpping it again then unequipping the crappier weapon then instead of the DPS with the Vampric Axe equipped showing as 11.09 as it should it changes to whatever the DPS was when only the crappy weapon was equipped.

If it is only the crappy weapon I am equipping and unequipping nothing happens and it always shows the correct 11.09 DPS value it's only when I have both equipped and unequipp then equip again the Vampric Axe before unequipping the crappy weapon that it will show the incorrect DPS value.

I've tried it with all the different junk weapons i've found before selling and about half of them will have it show this incorrect DPS value for the Vampric Axe.

I have tried it without the Vampric Axe using different combinations of the weapons that do and don't cause the issue but they have also displayed the correct DPS value it only happens when I use the Vampric Axe. I have yet to try it wither another weapon that isn't junk to see if it is only related to the Vampric Axe (or only this version of the axe I have) or other better quality items as well or if it also happens when using the axe with another non junk quality item etc.