I'm not sure what the precise language design reasons were for the differences in meaning, but I can help clarify how they are different:
In the first case: GeoGraphics[Locator[x]], the kernel does not know that you want to dynamically update the value of x, so, while it updates the coordinates of the Locator, it does not update x.
In the second case GeoGraphics[Locator[Dynamic[x]]], it does update the value of the variable x.
Here is a demonstration to clarify:
DynamicModule[{x1, x2},
x1 = x2 = {-63.`, 45};
Grid[{{GeoGraphics[Locator[x1]], Labeled[Dynamic[x1], "x1", Top]},
{GeoGraphics[Locator[Dynamic[x2]]],
Labeled[Dynamic[x2], "x2", Top]}}]
]
