LCTitan, the short answer is "NO," you don't get 30 bottles from a 6 gallon kit. If you do the math, 23 liters (what we theoretically start with including sediment) will give you 30 2/3 bottles at 750 ml per bottle. Now, if you can get less than 500 ml of sediment (good luck with that), you could end up with 30 bottles and change. When yields of 31, 32+ bottles are mentioned, the "top off" wine is not considered. I believe that anything over about 29 bottles from a white kit or 28 bottles from a red kit are just the "return" of the top off wine (or whatever was used to top off).
I used to puzzle over the same thing when I would see yields of 32 bottles while I was getting 27 or 28. But I was not topping off. I would start with a 6 gallon carboy, rack to a 5 gallon and a 1 gallon jug, then to a 5 gallon and a half gallon jug before I was ready to bottle and I would end up with 27 or 28 bottles.
Incidentally, the Italian made "6 gallon" carboys are really about 6.5 gallons, while those from Mexico are closer to a true 6 gallons. If you are racking only clear wine and topping off with a similar wine, you should end up with almost 33 bottles at bottling time from an Italian carboy or just over 30 bottles from a Mexico produced one. Also, when quoting the volume of a carboy, it is not clear to what point on the carboy is measured.