What exactly is your question? Is it "are % stats better than flat stats"? If so, yes, of course they are. Or is your question "are people finding they get more items with flat stats than with % stats"? In that case, the answer is no, the probability of each is equal, so you're no more likely to get flat stat items than you are to get % stat ones (excluding primary stat top-row / ring / amulet / banner, of course).

I know it's better o have % stats (like 3-4 times better in absolute terms).

The question was: the probability of getting any primary stats is equal? Do you have the same chance of getting a %ATK, ATK, RES and so on?

Yes.

Just to be clear, since you don't seem to be distinguishing between top row and middle row gear, all top row gear is flat stat.  The chances of flat/% hp, def, ATT, on middle row are equal

Angwil, of course I distinguish between top, middle and low (accesories) gear. And I know that In the top and bottom rows you only get flat primary stats.

My question was if the chance of getting (in middle row) ATK vs %ATK (or DEF vs %DEF, or HP vs %HP) was the same. I asked this because have in my inventory much more "flat" than "%" middle row gear. Maybe it was just bad luck ...

And my other question was if someone had done the maths of how much better was % vs flat gear. My initial estimation was 3-4 times better. 

You should not have any flat atk, def or hp gear from the middle row in your inventory. Sell that stuff, you will need the silver to upgrade your %-gear your champions should wear.

I don't know off the top of my head, because I never upgrade flat stats at all. But it's quite significant. There's really no reason to do the estimate though - because you should never* use flat stat items.

*the only exception being when you have a very specific requirement, like Stoneskin.

Yes, it was just curiosity of how much better/worse was one gear than the other, but I never suspected that  the "flat" one was 3-4 times worse (talking about primary stats, secondary should be equal). 

Secondary is far from equal. Flat secondary is significantly worse than % secondary.

It's kinda normal that if you dont cleanse your inventory often you will end up with more flat bottom pieces than % ones in it. The reason is simple. The % ones you use and equip on your champions, the flat one you don't, so while you deplete one pile by equipping them, the other pile gets bigger because you never use them.

Yes, of course, I didn't express myself well, sorry.

I took into account "equipped" gear.