Read the footnote in the legend (bottom left). Toronto controls 11000 to 27000 across letter A.

Here's the airspace in question. Believe me now? 😏

Be my guest and ask 😄 Here's an example of tiny overlaps between sectors on the west coast with Oakland and LA. Don't have FR24 premium otherwise I'd check how they depict this area, too.

Not an error. This shows you sectors that overlap between neighboring facilities. Not 100% on the details but specific altitudes will likely be controlled by Toronto or Cleveland here.