Question: isolve won't give all solutions here

Why won't isolve show all the solutions here?

> a := 6*x+2*y >= 48:
> b := 6*x+2*y <= 60:
>
> with(plots):
> aa := implicitplot(a, x = 0 .. 10, y = 0 .. 5, filledregions = true, transparency = .5):
> bb := implicitplot(b, x = 0 .. 10, y = 0 .. 5, filledregions = true, transparency = .5):

display(aa,bb)

We can see the region of solutions.  But if we use isolve (there is a small finite number of them), maple gives us only some of the answers.  Why?

isolve({a,b,x>=0,x<=10,y>=0,y<=5})
                                       {y = _NN1~}, {y = 5-_NN2~}, {x = 8, y = 0}, {x = 8, y = 1}, {x = 8, y = 2}, {x = 8, y = 3}, {x = 8, y = 4}, {x = 8, y = 5}

solutions for x=9 and x=10 are missing why won't maple show them?  allvalues won't work.  and what is NN1~?