You realize that n * 100% = n right?

That being said...Strava operates on many different data points including topographical data and data sets supplied by various riders with an altimeter. The altimeter data, I would assume, is manually inspected and only on an area of concern. Say a very popular segment that is literally ridden by hundreds of riders and say 10-20 people flag the elevation data there -- a person may look at the data gathered for the segment and alter the elevation correction data. Let's be honest, topographical maps are not very accurate for a newly paved bike path, for example, but may be accurate enough when determining how much higher above sea-level a town is than the nearby river and/or drainage system.

That also being said, it is a very simple question.

Divide the rise by the run. Multiply this number by 100 (not 100%) and you have the percentage slope.

You can't change that calculation ... your question is related to where Strava obtains their data points -- which is a question for them specifically...because no one is going to answer that but them for a specific location.