turns question

I just had a near near experience

i calculated my turns while under the effect of a levitation potion, and i had 1.17 turns of levitation. I moved and the potion wore off and i was only saved from disintegration by a ring of elements.

I had nothing slowing me down, and have a +5 amour of swiftness, so I'm not really clear why the potion wore off? Screenshot of equipment attached...