Differences in volume between analogue and digital outputs

[cmr_for3]

I noticed earlier I had to turn the volume up on my desk amp when using a Chromecast Audio connected via optical compared with when I use it connected to my PC via RCA and DAC.

I notice it but to a lesser degree in my living room setup with a Chromecast Audio also connected via optical.

I'm just curious as to why. It could of course by the Chromecast Audio making the difference....