The withdrawal fee you pay is always the fee a 1 kB transaction would need (hence "you pay the per kB fee").
If your transaction is bigger than 1 kB (eg because there are several inputs used, which quickly amount to a huge transactions, as they are way bigger than outputs), then the site takes up the remainder of the fees.
If your transaction is smaller than 1 kB, then you end up paying more fees than actually used.
As users can't influence how big their withdrawal transactions are, and it's hard for the site to predict how big those transactions will be, before actually sending it,
all users are paying the same fee, regardless of transaction size. This way, the system is fair for everyone.
Ok fine. Then my thoughts are that assuming a transaction size of 1kb is unrealistic (most transactions are much smaller than this), I believe that crypto-games would be definitely net ahead on charging fees this way. For perspective, Yolodice allows you to withdraw for only 15,000 Sat (regardless of the network), they probably make a loss on that but for people like me it keeps us happy. Well, I guess if no one else is complaining then the system will remain as is, however I think it'd be good to simplify it to change a static fee, where on average, the site is on balance even. I will take it that for all the other coins, the slider fee is the ACTUAL fee too.