Hello person!
First, thanks for sharing that site; it gives a lot of deep information to sink my teeth into. That said, I find myself questioning how the information is presented; especially for the varying lanes. Going solely by it however you need to look at the pick-rate and ban rate; about 29% ban and 4.7% pick. She's a popular champion but is open a /lot/ of the time and is just not being chosen.
This is because of how Akali works; she's not universally powerful unlike 'general' OP champs. She loses a lot of matchups and struggles in several comps, it takes effort to make it work. When you hit Plat+ soloque pubbing, Akali is only going to get picked in matchups that favor her to a point (or at least have some favoring bans). She's not getting an uneven win-rate because of raw strength (or her banrate would be higher), she's getting a winrate because she's being picked into advantageous situations.
And as I said; she /is/ a pubstomper (and a noob stomper, but not relevant in these %s). If you looked at her premade statistics, she fails. Fails hard. Akali gives very little against organized teams and, in that realm, would be straight "UP". There's a lot of work getting her to work in either situation.
When I said 'noob stomper' I meant that this isn't an issue in lower tier games at all. If I'm blowing through bronze/silver for gold at the end of the season, I can safely FP Akali every game without much risk of losing. The enemy mid will just let me farm/roam/etc and it'll be a slaughter; though actual bronze/silver Akali's don't tend to get those results reliably.
For counters... more or less, there are champions that should force me back before level 4 no matter how safely I play. That I should have <half their CS without getting ganked. It's really a one-sided punch-fest, but the people in these tiers don't know how to do it (or buy oracles lens. Recent nerf hit her HARD)
/Akali Tangent
(For the record, I'm mostly cheesing Vlad/Syndra/MF now because they're way more reliable at breaking the game)