Dell N4032F/1548P Light Level Question

Hello,

I'm experiencing some odd light level results on some of my switches. Specifically I'm wondering how it's possible to have a higher input power than my output power. Even assuming there's no loss this doesn't make sense to me. Is it something to do with the way dBm is measured?

              Output/Input 

Distro    -1.142  -2.917 dBm

Access  -0.870  -0.919 dBm

Even when doing a loopback test, I get output power of -1.835 dBm and input power of  -1.600 dBm. 

There's no service impact here, I'm mostly just looking to learn.

Thanks,

Peter