The "Notes Regarding Intelligence Table II" in the PH is not just for determining which spells you "know", it also is intended to serve as the means to determine what spells are actually in a casters spellbook. The process of checking to "know" spells is repeated for each level of spells as the caster gains access to them by gaining levels.
It does little good to find out how many of the 30 listed 1st level spells your PC "knows" if you then still have no idea what spells are actually going to be in his spellbooks. You can "know" all 30 1st level spells and still be scratching your head as to what spells you have in your books and can actually study and cast. The key sentence is:
Percentile dice are rolled, and if the number generated is equal to or less than the percentage chance shown, then the character can learn and thus know that spell (it may be in his or her spell books - explained hereafter).
I'm guessing here that people are emphasizing the idea that the spells MAY be in your spellbooks - that is to say, they COULD be but just because you "know" the spell doesn't mean you HAVE the spell in your book. But the intent was to create the contents of a spellbook for a 1st level magic-user by this process because without it there WAS NO process to fill out the contents of a new characters spellbook - only the
potential spell contents.
A year later, the DMG presented a COMPLETELY DIFFERENT (and vastly more limited) process for providing 1st level characters with their initial spellbook contents. Everyone now got Read Magic by default - you know it, it is in your spellbook, no roll to "know" the spell is required. Then the remaining 1st level spells are divided into three groups - offensive, defensive, miscellaneous. You roll randomly for a spell from each group (or the DM can elect to allow players to simply choose). Again, you then "know" these spells, you have them in your spellbook, no roll to "know" is required. ALL SPELLS ENCOUNTERED THEREAFTER require that the player roll to "know" them if he wants to transfer them to his spellbook except spells which the character personally successfully researches.
The PH method provides a good repertoire of spells for the duration of the game and remains commendable for those who don't care to muck about with the process and are more generous to wizards. The DMG method gives the player a very limited selection to start and then actually give the DM greater control over the "quality of life" for wizards. He can let them choose the intial spells they want and then control what variety of spells and how many they encounter in captured spellbooks and scrolls to potentially add to their books. The recommendation therein is that players be allowed to swap spells freely between themselves but that NPC's and henchmen be the biggest jerks possible about it (thus overcharge disgustingly for spells the PC may want). Both retain the potential for a character to fail to "know" key and highly desired spells.
