Thats just how it is, the bell housing is a different shape . So the DFY won't fit as nicely as on a LCP or LUG and even on those there is a slight gap. But if you disconnect the focus Yokes and look at the grid, no loosen all the magnetics and slide the focus yoke up and down the tube neck to get the best focus in the center you can. With no power it should still focus ok, if not then your yokes magnetics are not of proper gauss and would need re-calibrating. or you could try to find some other yokes and by testing each one find the best yoke for each tube.
On my test tube I used to modify the VNB's for those tubes I got good results with the 8 inch DFY's the 9's are not much different in Shape, the coils might be slightly different. But otherwise people have had them work fin on other 9 inch tubes.
here is a picture Scott sent me, 8" on left 9 " on right. I think the restores are different but i cant tell from this pic.