Battery Full [Q] Does a usb powered battery "last less"?

I bought myself one of those portable batteries to charge my phone and tablet on the go. It always seems like when I charge my Nexus 5 from the wall charger the battery lasts longer than when I do it from my car usb, or the portable battery itself. I could get 90% from the wall charger and get to 90% from the portable battery and the latter seems to drain faster. Am I just am I imagining things?