You're not imagining things, there are various things that can effect the perceived level of the wet vs dry signal. Technically a mix of 50 is adding the dry at wet outputs at unity level. However, using two delays rather than one, the actual delay times, the filtering, and even some of the nonlinearities in the effects themselves can effect the actual or perceived levels of the sounds. So while we've made a pretty good effort and trying to equalize the levels across the board, it'll never be totally constant. The best advice I can give you is close your eyes and turn the knob, when it sounds good, stop.