The important metric for power supplies is power which is voltage times current. Identifying voltage only tells you half the picture, your power supply must also deliver the necessary amps (current).
Between 9 and 10 volts there isn't much difference so it doesn't matter much.
Some third party power supplies are made poorly (e.g. cheap and low capacity capacitors), in consequence they deliver less current, the closed circuit voltage is less (the voltage measured when the console is plugged in and working) and as a result they deliver less power.
These problems are exacerbated when you use a power hungry device (compared to regular cartridges) like the SD2SNES.