My guess is that the upgraded booking class was P or Z, which also earns 100%. So that would give 500. The C only refers to the cabin class, UA upgrades don't move in to C as booking class. Though I a long while ago, when I paid for upgrades on UA it kept the earning for the original booking classes. But I need to double check that.

The minimum is actually wrongly written I think. As the minimum is 300 miles at 100% for Y, and then the 600 goes back to when J was 200%. So should really be 300 and 450.

If I look at ANA the minimum Y is stated as 300 which is then 450 with 150% being minimum for premium classes at 150 it is definitely in line with how it's is calculated. However ANA has 25% earning classes, which would actually give you 75 points, and SK neglects to take this in to account.

Now, how they write should also be how they pay. It is clearly published, so they should cough up and fix the website.