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 modi

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.