Are you sure about that? I built a simple rocket with a Frontier engine and measured its delta-V (by burning all fuel with drag and gravity disabled):
View attachment 8924
I end up at 3825 m/s, which corresponds to a specific impulse of 285.7s
I can't explain the exceeding 0.7s (I should have ended at 3815 m/s instead), but besides that the given value seems correct...