Many seat types have multiple distinct motors for redundancy reasons. Even if there are not, you can adjust the effective output of a solid rocket motor by modifying the nozzle configuration, and Martin-Baker has an expired patent on this.
... but even if the seat in question was able to adjust the ejection thrust (which I don't think it was), this particular situation involved ejection really close to the ground, which calls for maximum thrust.
... but even if the seat in question was able to adjust the ejection thrust (which I don't think it was), this particular situation involved ejection really close to the ground, which calls for maximum thrust.