Strange behavior of hypergeometric function in maple

I cosider (finite) hypergemetric series for 2F1 satisfying Kummer's identuty at z=-1 . Depending on

1. evaluation it by hypergeometric 2F1[a,b,a-b+1]
2. or Kummer identity
GAMMA(a-b+1)*GAMMA(1+(1/2)*a) / (GAMMA(a+1)*GAMMA(1+(1/2)*a-b))

or using pochgammer symbol notation:

pochhammer(1+(1/2)*a-b, (1/2)*a)/pochhammer(1+(1/2)*a, (1/2)*a)

I get copletely different answers. Perhaps anybody knows what might be the problem?

CODE:

eval(hypergeom([a, b], [a-b+1], -1), [a = -2, b = -4])

OBTAIN -2/3

eval(pochhammer(1+(1/2)*a-b, (1/2)*a)/pochhammer(1+(1/2)*a, (1/2)*a), [a = -2, b = -4])

OBTAIN

-1/3

BUT (this is in fact Kummer's identity):

simplify(pochhammer(1+(1/2)*a-b, (1/2)*a)/pochhammer(1+(1/2)*a, (1/2)*a)-hypergeom([a, b], [a-b+1], -1), 'symbolic' );

OBTAIN
0