I suspect the Flaps 1 stall speed is more than 110% of the Flaps 15 stall speed. When you compare the maneuvering speeds at a nominal landing weight, i.e. ~150ish for Flaps 15 and ~180ish for Flaps 1, you're looking at about 120%.
Some aircraft just have slats extended on Flaps 1 setting and no flaps, not sure if this can be the reason.