Yes, it is mainly scar tissue....it also depends on the individual cat and the sutures used. It doesn't really matter whether or not disolvable sutures or removable sutures are used, the lump occurs as a result of tissue reaction. (although, sometimes it can be an infection relating to disolvable sutures). My cats were all spayed using either suture types, only one ever developed a lump that was simply scar tissue.
To better describe this, think of tissue reaction, providing that infection is not present, then when the tissue heals, it develops scar tissue, which can actually be a protective layer. It is usually not bothersome, but during recovery from a spay and if an owner notes that the lump is increased after 7 days post-op, it's time to get the kitty rechecked so the vet can rule out infection or if he had used removable sutures, to ensure that one was not missed during removal.
Another cause for the lump can be the vet's technique, whether he uses a vertical or horizontal incision. In my experience, vertical is best (but vets have their own techniques they are most comfortable with)....................Traci