There are two main factors in erg mode to hit a target power: resistance and cadence. The trainer is constantly adjusting the resistance to maintain the target power as you have natural variances in cadence.
Since power is essentially force x speed, sometimes if I’m not paying attention, I find my cadence drifts higher to reduce the force required of my legs (same force and higher cadence = more watts).
The best way I’ve found to deal with this is to gradually bring your cadence back down and let the trainer keep the power target. If you try to suddenly jump down to a lower cadence in erg mode the trainer is going to respond with a big and sudden adjustment in resistance, which can be killer at the end of an ftp test when you’re barely holding things together