What he think he means to say is that it really can't be explained, since it's the developers behind Fantasica who produce such stats for the cards.
There isn't any point in trying to understand why such stats are out of whack, since we have no control in changing it for the better. Even having a suggestion box on Mobage's site doesn't do any good for us.
If you wanted to, you can assume it's a marketing ploy from the devs themselves. Since they're always releasing new units every week to make sure people are still taking interest in buying packs, newer units like Minotaur 4* or Frost will start to have better stats than the older 5* cards like Fiona or Julia, just to force you into spending for the more expensive packs.
If you're not a spender, this will still affect in how you can get these cards through other players, since they won't be willing to sell at an easier price either.
You can also say it's a way of making sure you're not sticking with the old stuff and changing it up with some new loot constantly.