Very curious. I experimented with this behaviour and found a transition point where the operator form stops agreeing with the non-operator form:
Clear[pt];
pol = Polygon[{{{0, 0}, {10, 0}, {0., 10}}}];
pt[x_] := {-1.*10^(-x), 1};
Manipulate[
{RegionMember[pol, pt[x]], RegionMember[pol][pt[x]]},
{x, 5.77316, 5.7732}]
To keep safe, I would use exact polygons if possible.