CFM56-5B vs 7B reverse idle.
 

I've noticed on the A320 family, that when idle reverse is selected after touch-down, there is a very brief increase in thrust before the engine settles at flight idle. This doesn't seem to happen on the 737NG.

What is the reason for this?

It does happen on the 737NG as well; normally up to 43/45% N1 then below 40%. Don't know the reason but I guess is something to do with the inertia of the airflow changing direction rapidly.

Bleeds operating transiently perhaps?

On most Boeings, there is a five second time delay on touchdown before going from approach idle to ground idle. Depending on how quickly you get the reversers deployed, you may be seeing that transition...

You can hear it in this video.

easyJet Landing Madrid Barajas A319 - YouTube