By the house including the zero and double-zero on the layout and not paying you true odds for their inclusion. The casino pays all wagers on the layout according to odds that would be accurate if there were just 36 numbers on the wheel. But by adding 0 and 00, they produce 38 numbers. True odds of winning a single-number bet should, therefore, be 37 to 1, but winners are paid only 35 to 1.