Kids who master the material in Singapore math can do quite well on standardized tests. A kid who is doing well with Singapore 5 can easily skip 6 and move directly into prealgebra (or even algebra).
I wouldn't make a placement decision based on one test score unless that test score reflects what you already know about your child's achievement. That you're surprised about the score makes me think that additional testing might be helpful. However, for a strong math student, the advanced track you've described sounds like a good one.
