A lot of the talk about antibodies is very confusing to many of us, but basically "RA" factor is rheumatoid factor, an antibody which is present in the blood of most people with rheumatoid arthritis (RA), but can also be present in people with lupus (SLE or other forms of lupus). RA and lupus are separate diseases, but both are "auto-immune diseases" (diseases where the body turns against itself and begins attacking its own organs by forming antibodies against them). The disease can have very similar symptoms and are often treated with the same medications, but RA tends to affect smaller joints on both sides of the body first, and can cause severe damage and deformity, whereas arthritis cuased by lupus can affect almost any joint in no particular pattern.
Many people with lupus can have what is called an overlap disorder, where they have symptoms of two or more connective tissue diseases like lupus and RA. Lupus patients can also have rheumatoid factor in their blood without actually having the symptoms of RA.
So, yes, it is very common for people with SLE to not have a positive RA test, but there are also many people who do. It really doesn't affect your lupus diagnosis.