carb ported and manifold vacuum question

No, this isn't to ask which one to connect to my vacuum advance, but it is related (I was searching on the web and that subject is as bad as politics and religion!)

I was looking at my tuning and, in the course thereof, hooked up a vacuum gauge to the manifold vacuum port on my carb and the ported vacuum port on the carb. The manifold vacuum was giving me 4 in Hg at idle and the ported 10 in Hg at idle. I double checked my vacuum gauge with a loaner vacuum pump from Advance and it checked out fine. So, I double checked my measurements today and got 6 in Hg and 12 in Hg from the manifold and ported vacuums respectively.

The carburetor is an Edelbrock performer model 1400 (the 50 state street legal model) and the motor is a 340 with Edelbrock performer RPM intake and comp cams XE274H cam. All of the carb documentation said that ported vacuum should be nearly 0 at idle and the Edelbrock tech line guy said my measurements were impossible.

I checked, as best I could without taking the carb off, that the throttle plates were moving and such and they looked normal. I didn't see any obstructions or anything in the carb. Has any seen this or have any suggestions on what to look for or what it means? The car has been dieseling a little bit when I shut it off so now I'm wondering if the two could be related.