Your interpretation is exactly what I *wanted* the MM to say, and what I could not in good conscience rule that it said.
The text reads "All dragons start with 6 skill points per Hit Die, plus bonus points equal to Intelligence modifier X HD".
So an Ancient white dragon (30 HD, Int mod +2) has 6 skill points per Hit Die (6*30=180) plus bonus points equal to its Intelligence modifier per Hit Die (2*30 = 60). So a total of 240 skill points, three of which per hit die are spent on required skills, leaving (3*30=90, 240-90=150) 150 skill points left to spend. Plus the 30 bonus ranks of Spellcraft.
Note that the MM does *not* say that all dragons start with (6+Int mod) skill points and gain (6+Int mod) skill points per Hit Die beyond the first, which would imply your interpretation. I urge you to look at the semi-official stats I linked to above - that dragon has skill points calculated similarly to the above, i.e. retroactive skill point gain for Intelligence increase.
This goes along with the free ranks in Spellcraft - even per your table, an Adult white dragon has 0 free ranks, while a Mature Adult suddenly has 21 free ranks. Retroactive skill point gain has exactly the same effect - a Mature Adult white dragon, upon gaining an Intelligence modifier of +1 (previously +0), gains an additional number of skill points equal to its Hit Dice. While Spellcraft as a "special ability" does make sense, the two seem to go together mechanically.
Also note that a Mature Adult white dragon is technically a different monster from an Adult white dragon - or at least it's given stats as if it were. A Mature Adult is *not* a hit-die advanced Adult, which would follow normal rules for HD advancement (i.e. no retroactive skill points).
I would *like* to disallow retroactive skill points for dragons. But the more I look at the book, the more convinced I become that "dragons are special".