May be different at your school but mine says: "A studentís percentile ranking is easily calculated by merely dividing the ranking, and always rounding up to the next whole number percentile. For example: if a studentís rank is 86/250; dividing 86 by 250, yields 0.344; which means the student ranks in the top 35th percentile of the class." So, I would assume they would round 15.4 up to 16 even though its .432. But if the rules are stated diff. in the handbook maybe you can argue it, ours was stated that way in an e-mail that came out with rankings.
What does Top 15% mean? Mean 15.00000... and lower. I would imagine that your handbook elsewhere has a rule regarding rounding. Or your career services probably does for how to put ranking on resumes. Rounding to the closest whole number works for estimating, but not for percentiles, which are predicated on coming "under". Top 10% is everyone 10% and less. So the cut-off is naturally on the whole number.