I have a SYN5050 and thought the same thing at first.
I was using my RM4 with the power section of my 50w 5153 and it was stupidly loud, I was expecting the same volume on the SYN5050 when I got it but I got the same impression as you did.
I think that what happens is that the power section on the 5153 (or other heads) is all open and you only control the volume through the master output on the RM4 (M4 or SYN2), whereas if used with the SYN5050 you're controlling the volume after the RM4, so to achieve a similar stupidly loud volume you'd have to have the volume of the SYN5050 turned up all the way.
The other thing I found out is that when you use the SYN5050 in 100w bridged mono, if you have the channel B on standby the volume remains at the same volume as the 50w stereo mono (50w/channel), you need to switch ON the channel B to hear an increase in volume (which is not double the volume as we already know).
Can you confirm your unit behaves the same way?
Not sure if that is the way it is supposed to work or not but that is how the unit behaves.
Hopefully somebody from Synergy can comment on this?