Not sure, but "mysteriously" I seem to miss all of my OG's when I use a Midwest kit as well. Funny, it doesn't seem to happen with other kit providers like AHBS and Williams. Maybe it is just the cynic in me, but just maybe there is a reason the Midwest kits are constantly cheaper than the other guys...
So, now I just add to my Midwest kits to reach my goal OG. For instance, I bought their standard Porter kit and added 16 oz, of my own honey and 1 lb. of Crystal 60. Even with the addition of the honey and grains, I came out with 1.046 OG - which is the OG they state their standard kit produces. Hmmmm.
In all seriousness, I really don't care how a kit is packaged for a company to meet their margins (after all, they must make money to stay in business) - I am more concerned about truth in advertising on what I am buying. After all - on a pre-measured, pre-packaged beer kit, you should pretty much hit dead on with all your OGs every time.