I invested some and hope to gain some profit.
I invested in period of two weeks and earn about 3% from it

. I withdrawn already.
You're luck there was no high-rollers hitting your investments.
Yes, you are right that there needs to be luck too. It's still gambling related investment. I am sure that there were high rollers in time I put Bitcoins there. You can see your actual investment and profit all time. And it went down and up. But there is a House edge and this works well. Even high rollers will lost some day.