Electrical Sub panel unequal legs
You have a lost neutral. Serious business! Fix immediately!

I don't know whether you've lost a neutral in a multi-wire branch circuit, at the subpanel, at the main panel, or at the pole.  The most common one is "at the pole", which is good news because that's the power company's problem and they'll fix it fast for free. Just report a power outage, which is what it is. 

Once I had this happen on a 30A feeder which supplies our cottage.  Checked the other house circuit, nope, it's low too. Then I discovered we were only 1/2 of a 30A MWBC with the next cottage over, who was getting ~150V and was frying appliances.  Suddenly her complaints of frying appliances made sense.  So it's the creaky MWBC to the cottages - nope, it's the ancient FPE master panel.  Nope, it's the meter.  Nope, it's the pole - our entire complex was affected and the neighbor too.  Moral of the story: cast a wide net when searching for this thing. We should have started at the master panel.  

What's really happening

Look at my diagrams here that describe how split-phase works.  What  you have going on is this: 



Notice how the 240V loads don't care, but the 120V loads are now out of balance.  That's what is happening, and it's changing as loads are turned on and off.  The two 120V legs will add up to 240V, but  they won't be 120V.