Could be noise abatement as mentioned, there are monitors near airports so they often throttle back as they climb over them, it used to have the opposite effect at my local airport as the older noisy stuff used full power for take off the throttled back as they went over them before opening up again, so those in the local vicinity actually got more noise as they climbed on take off the gain height speed at higher power settings than actually needed. I would also ask on pprune.