Individual results will vary, but the RIAS has a general reputation for under-representing IQ, because of quick discontinue criteria. Basically, if you miss two questions in a row, you've hit the ceiling, and they move on to the next section. The test is designed for brevity over accuracy.
Mathematically, it makes sense that if a number of people are hitting the quick-discontinue and having their scores deflated, there's a tiny subset that isn't, who experiences significant score inflation as a result.
