if dicebitco.in works similar like just-dice I think your math is wrong, because there is a 10 % fee on investment profits.
(probably in order to pay for hosting, security and so on)
I forgot to mention commission, you're right.
Everything I wrote is true, BUT at the end of each week the site takes 10% of any new net investment profits from you.
As I understand it, DB works exactly the same as JD in that respect - if you lose one week and win it back the next week, you don't pay any commission on the part that recovered your losses, etc.