See the code from the REPL below. What gives?

```
julia> using SymEngine
julia> @vars x1, x2, x3
(x1, x2, x3)
julia> f = sin(x1*x2) + exp(x2 + x3) - x3
-x3 + exp(x2 + x3) + sin(x2*x1)
julia> gf_x1 = diff(f,x1)
x2*cos(x2*x1)
julia> gf_x2 = diff(f,x2)
x1*cos(x2*x1) + exp(x2 + x3)
julia> gf_x3 = diff(f,x3)
-1 + exp(x2 + x3)
julia> gf_x1(1.,2.,3.)
-0.416146836547142
julia> 2.0*cos(2.0)
-0.8322936730942848
julia> gf_x2(1.,2.,3.)
53.76585636005
julia> cos(2.0)+exp(5.0)
147.99701226602946
julia> gf_x3(1.,2.,3)
19.0855369231877
julia> -1+exp(5.)
147.4131591025766
```