I'm not saying they've not favorded higher discounts on some items... :)
They could be using some different random function, for instance, though it's obviously a Gauss curve, considering the (existing and attested) 10% and 90% discounts rarity (maybe there are even 5% and 95%, people having gotten them could confirm).
Now gauss curves have parameters. :) The central value probability could be raised or lowered, affecting the values close to the ends as well.
But honestly, I've had as many 80% as 20% on all items, etc. and I can affirm 40%, 50% and 60% (on anything) are by far the most frequent.
I've been playing from before the Wheel of Fortune was added, so I've had plenty of time to realize it's a simple Gaussian curve. :p
In addition the number of sales is random as well. I get 200 or 250 x 40%, 60% or 80% discounts or revivals often enough, for instance, but yesterday I only got 150 x 20% discounts.
Now combinate the discount percentages, the number of items and the items nature, and you get quite a lot of possible discounts everyday.
Completely random, I tell you. :)
Of course, computers usually use pseudo-random numbers generators rather than hard wired (and expensive) random generators. They use operations on numbers giving apparently random sequences of digits, but could ca hardly prevent some similar sequences to reappear. However, consider the generator is checked at different moments when different people are logging in, with different pllings for each of them, and you'll get quite satisfactorily random combinations.
Anyway, biasing the random generators with code to advantage Plarium would just be uselessy and hugely complicating things for puny results.
Remember the discount percentages probabilities follow a Gauss curve already, meaning average values are much, much more likely to drop than extreme low or high values.
This is quite enough to be fair for both Plarium and us.