All, Take 25 B-nova's out of the box and set them up and see what sort of differences you get between guns, the reality is that there will be differences in them, they will not all shoot six inches high or low or left or right. The trick is to find out where YOUR gun hits, which you already have, you claim they shoot high.
You have 2 choices, aim low, or adjust the POI.
Here are 2 good web articles that will give you some more information. There are tons more avaiable.
http://findarticles.com/p/articles/mi_m0BQY/is_7_49/ai_102274467
http://www.huntingmag.com/guns_loads/why_pattern/
As mentioned, the point of impact can be adjusted if you know what you are doing but it can be a bit frustrating if you don't. Furthermore, I'm not sure if B-Nova comes stock with shims to do this or not but I know it would be difficult to shave a plastic stock!
Also, how you mount the gun, where your check touches the stock and how you look down the barrel can have change POI. This is why if you have a Nova and I have a Nova and we both shoot the same everything we might have two different POI's, shells, pellet size, barrel length don't matter.
I have an 11-87 that I've worked and worked with, like shooting Old Rem Hevi-shot out of it, it shoots low and I just aim dead on the birds head instead of lower, like jlo1 points out, this isn't optimal, but I've fired well over 150 turkey rounds through this gun and I love the pattern that I get at 40 yards. It hasn't failed me yet.
If you are more serious about this you could get a gun with an adjustable stock.
Finally, just to share, this past year I purchased the Rem 870 with the thumbhole stock and raised 'rifle' comb and it is a dream FOR ME (others might not like it). I've yet to find exactly what shell I want to shoot but Win. Supreme#5's 3in. Magnum Turkey did the trick last year quite well and I didn't have to aim high like with the 11-87.
I think getting your turkey gun to shoot is one of the most fun parts of the whole thing.
POI is the #1 most important aspect of shooting the scattergun and having it hit where you want it to hit!
Sorry for rambling, good luck!