Epidemiology equilibria solved, but what happened??

edit

var('beta gamma S I R N')

dS = - beta * S * I / N dI = beta * S * I / N - gamma * I dR = gamma * I

solve([dS == 0, dI == 0, dR == 0], [S, I, R]) [[S == r7, I == 0, R == r8]]

Why am i getting different values for S and R which correlate to how many times I run the code? At first I thought that it was because I did an interpolate above, but when I went and modified the code a bit at the beginning I was still left with increasing equilibria values.

the next run will be [[S == r9, I == 0, R == r10]] Does anyone know why this is happening??