So firstly, the higher your level, your stats divide by a higher number. In other words, they get weaker and you need more Skill Speed to achieve 0.01 speed. However, your overall stat amount syncs down to compensate for this. It's possible the exact discrepancy in this results in it being different.
However, another factor is at play. Materia does not count when synced. Instead, the uncapped secondary stat on your gear gets capped. So if you have 200 Skill Speed and 270 Determination on a piece of gear, the Skill Speed gets bumped up to 270, rather than it syncing your materia.