They all said his speed scares them (Hattori said this twice specifically), he's supposedly very fast. 10.98 is still a whole lot closer to 11 than flat ten, so I'll take that for now. In a sport like track, fractions of seconds and millimeters count, so something that may seem small can still be a pretty big deal. Like I said, he's only .02 seconds away from 11 seconds, which is somewhat more realistic for high school students.

Here's a high school record from Ohio:

100-Meter Dash 10.48s Mario Allmon, Cincinnati Princeton Dayton 1990

So actually, Yamato's time isn't really all that unrealistic in the first place for someone who is proportedly very fast.