The difference is in the denominator in the equation that determines the edge. Say you bet red in a game with two zeros. There are 36 numbers plus the zeros, making a total of 38 slots on the wheel. Of them, 18 (the reds) are winners and 20 (the blacks and two zeros) are losers. That's a result of -2 units in 38 plays, yielding the equation -2/38 = -5.26%. On a single-zero wheel there are only 19 losers, meaning you lose only 1 unit, which would seem to indicate an edge of -1/38 = -2.63% (as you suggest). However, removing the second zero leaves only 37 total slots, so the equation is -1/37 = 2.7%.
