Her sire is a tobiano and on top of that she has the spotting on the coronet band. That is one of the requirements of a tobiano. It is not uncommon for a tobiano to throw what APHA calls a slipped tobiano solid colored foal but with the spotting on the coronet band. All of Sky's solid foals have this trait on them.
We are in the process of having Sky tested as a homozygous tobiano stallion. He comes from homozygous tobiano lines. APHA also told us that a homozygous stallion can throw solid foals but they will have the spotting on the coronet band.