Those results seem somewhat reasonable. A 10 ppm increase is about 18 lb ‘available’ P/acre to 6 inch depth. Whether the soil extractants used reflect what is actually available is another story but let’s assume it is. Cover crop data from one of our university research farms shows cereal rye aboveground P uptake is ~10 – 15 lb/ac. If we assume that that P is now plant available whereas it wasn’t before (an assumption I know but fairly reasonable) you are getting in the ballpark of the numbers you are seeing. The above numbers are just aboveground P and don’t account for the contribution of roots and increased biology from the cover crop, which would also help increase the available P number.