League-of-legends – How are champion stats calculated

Those stats are an approximation of a champion's overall ability, and do not imply any base stats in particular. The game does not use them for any calculations. In general, a champion with a high "Attack" bar will be expected to have effective autoattacks (or will deal a lot of physical damage through abilities), one with a high "Health" bar will take a lot of hits, one with high "Magic" will deal a lot of damage with abilities, and one with high "Difficulty" may take a little longer to get used to.

In particular, "Attack" includes things like Base Attack Damage, Base Attack Speed, and any abilities that enhance basic attacks, such as Tristana's Rapid Fire or Fiora's Riposte. It also considers abilities which deal physical damage, so AD Casters like Pantheon have a high rating as well. Since champions tend not to deal much damage without being in range, Gap Closers (like Rengar's Unseen Predator) or a high Attack Range also may contribute.

"Health" includes not only a champion's Base Health, but also their Armor, Magic Resistance, and abilities that protect or sustain one's self, like Shen's Feint, a self-shield.

"Magic" usually represents either high magic damage with abilities over a period of time, such as Ryze, or high burst damage with abilities, such as LeBlanc. Thus, it considers not only the Base Damage and AP/AD Ratios on each ability, but also cooldowns and mana costs (or lack thereof).

"Difficulty" is very rough, though champions with more skillshots (like Ezreal) and champions with unique mechanics that can be hard to master (like Orianna) will have a higher rating. Interactions between abilities also can drive up a champion's difficulty. A champion with high "Difficulty" may have a high skill floor (minimum skill required to begin to be effective), a high skill cap (maximum skill before further improvement with the champion is no longer possible), or both.