to the leachate from a drip to waste system.

I assume that you're measuring nute ppm @ the return/overflow line to your res?

Evaporation of the water in nute will concentrate the salts remaining in the water.

Suggests that your plants are transpiring more water than they are absorbing nutrient salts, though evap of solution remaining in tray will also concentrate salts.

If your swing is very high, you might want to irrigate more frequently.

It would be helpful if you could describe your measurement technique in more detail.
For instance, if you are measuring the first few drops of return, I would expect a higher concentration. In this case, if average res EC was relatively constant, it wouldn't concern me much.
It would also be interesting to know the relative temperatures of res and return flow, as not all meters will compensate for conductivity/temperature.

I've always found salt concentration to be a problem in E&F systems based on a rockwool substrate- the hydroton used alone is much more stable. If you're going to use RW at all, it's best to completely submerge it each flood cycle, to avois salt buildup on RW surfaces which wick up solution but are never fully rinsed.