Point of origin is irrelevant to where the caster stands and casts the spell.
Technically, you are correct. The point of origin can be anywhere within the range of the spell for many spells. For some spells, like ray spells though, I think the point of origin is by definition the caster. So the question is whether something like the following could happen: a wizard completely within an antimagic field then attempts to cast charm monster on a monster outside the field.
First of all, given the way magic is described throughout D&D, the caster is inherently tied to the magic of his spells. So it flies in the face of logic to suggest that the above example would be possible. Appealing to logic doesn't always convince people when talking about rules though, especially when the rules are so tied up in specific terminology.
Second, there is a lot of evidence that the designers never intended to allow antimagic field to let casters cast spells within it. The Initiate of Mystra feat certainly seems to be designed for casters who find themselves within an antimagic field. But it could be interpreted to mean the caster with the feat can just create magic with a point of origin in the antimagic field.
This is entirely anecdotal, but when I played in Living Greyhawk (I was very active in the community, probably over a thousand hours of gameplay as both a player and judge), I never saw any judge allow a character to use any magical ability if the character was inside an antimagic field at the time. And I played all over the United States in at least a half dozen states. I am sure there are people with different experiences though, which is why threads like this even exist.
So what's the answer? I can't really tell you with 100% certainty because there seems to be some ambiguity written into the spell and an almost intentional avoidance to address the specific issue of whether a caster can or can't cast a spell while inside an antimagic field. You'd think it would be a fairly straightforward question to answer and one that would be included in the spell, but either the designers thought it was obvious the way it was written or intentionally left it void. I am leaning towards the former, but then the argument just boils down to the true intent, which is hard to discern without Monte Cook, Skip Williams, or someone similar chiming in on the subject.