More on this...
However:
Chance to Know Each Listed Spell pertains to the percentage chance the character has by reason of his or her intelligence to learn any given spell in the level group. The character may select spells desired in any order he or she wishes. Each spell may be checked only once. 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). Example: A character with an intelligence of 12 desires to know a charm person spell that he finds in a book or scroll, percentile dice are rolled, but the number generated is 52, so that spell is not understood and can not be used by the character
At that time. Nothing is said (either way) about later attempts at a different level, at different intelligence, or even just off of a different scroll.
Now, read the Minimum Number of Spells:
Example: The magic-user mentioned above who was unable to learn a charm person spell also fails to meet the minimum number of spells he or she can learn. The character then begins again on the list of 1st level spells, opts to see if this time charm person is able to be learned, rolls 04, and has acquired the ability to learn the spell. If and when the character locates such a spell, he or she will be capable of learning it.
This is still only referring to that initial roll-through*.
Together, they mean that you get one shot to learn a spell; if you fail, you can never learn it again unless you increase your intelligence or fail to have the minimum number of known spells.
I don't read it that way.
* - this initial roll-through is something we've never done, as it in fact turns out to be an unnecessary and redundant step (actually, come to think of it the whole idea of minimum knowable per level is redundant).
At first level you get Read Magic (automatic) plus one random spell off each of the three lists as shown in the DMG (I almost always give a 5th completely random spell, this is a DMG option). That's it. After that, there's no need to roll comprehension until-unless a new spell is encountered - be it on a scroll, in a spellbook, traded from another MU, or wherever - at which point if the player/PC decides to try to understand/learn the spell a roll is given.
So, if a MU starts out with Read Magic, Charm Person, Hold Portal, Identify and Featherfall then later encounters Magic Missile on a scroll and tries to copy/learn it, only then is a roll made to comprehend MM.
Let's say her max limit is 9 spells per level; she gets MM, so now she's at 6 firsts - lots of headroom.
Next adventure she finds a scroll that has Nystul's Aura, Detect Magic, Tenser's Disc, Mending, and Write. She can only end up learning three of those before she hits her ceiling, and might even want to leave a spot open in case she ever finds this Sleep spell she keeps hearing about. So she prioritizes the five on the scroll, and starts studying and copying. She blows Detect Magic, then succeeds on Write and Tenser's Disc and is at 8 spells known.
Now, she's got a choice to make: does she keep that last spot open for Sleep, or does she go for Mending right now?