It's probab;y cable loss - the down side to true bypass can be a loss of volume and bite when effects are off, as the guitar has to drive long cables through the FX board and beck to the amp, without he help of a buffer found in non true bypass effects.
Pete Cornish has a great rant about true bypass on his website - worth reading! I like one buffer on my board - usually my Fulltone Choralflange. Keeps the tone constant - but too many buffered effects thins the tone imho!
