Being on cycle may have you pounding-out reps faster in a more explosive way -- that's my quess.
As mentioned earlier, vertical jump = power, which is Force/Time. We normally focus on the "Force" component (i.e. How much can you squat) but often ignore the time component. If you want to develop more Power, you can do a few things such as:
1) Plyometrics -- pre-streach a muscle before you use it. Box jumps and bounding hurdles are probably the two "classic" approaches
2) Focusing on time -- While squatting (for example), keep your eccentric phase slow but once you bottom-out, focus on speed. Certainly don't jerk the weight, but pick a squat weight where the bar is gaining speed as you finish the lift as opposed to "grinding it out". You may have to cut weight, but 6-8 controlled, explosive squats with a strong finish can do surprising things for your vertical.
Interestingly enough, a re-emphasis on power can be a good way to push past a plateau as well. I often get into these grinding contests on bench press for weeks and weeks then sit around bitching about not seeing any progress. If I back off weight a little and focus on 6-8 explosive reps, sure enough I can inch past the limitation (normally).
P.S. If explosive stuff kills your joints, a little deca goes a long way. My 37 year old joints are lubed-up nicely now!