Modulo constraints on space, I'm not sure what people's reasons are for inviting someone to the reception but not the wedding.

The actual wedding is, for at least some people, more a legal formula than a ceremony-as-such; the reception is a party to celebrate. If you view it in that light, you may actively not want to invite people to the boring legal bit, while still wanting them at the fun party afterwards. (In addition, as you say, to space issues.)

j4: what everyone else has said. I thought the invite was perfectly clear, and must remember to actually RSVP.