With me, I believe it's entirely psychological, and mostly has to do with tenseness. I can go into a singing session or weekend of music with a fresh voice, thinking I sound like crap. At the end, my voice is worn out, but I sound better to myself.

There are times when I can hear every slight (and not-so-slight) off pitch note I sing and it drives me nuts because I think I'm physically incapable of singing on pitch. I AM probably singing as close to on pitch as most decent singers can manage, but my brain somehow thinks it's reasonable to believe I can be perfect.

Do the tape thing that Morticia mentioned. Maybe you could make two separate tapes and not label them. A week from when you tape the last one, have a listen, and see if you can even tell which is which.

If you actually DO sound different, there are so many factors that can contribute. Allergies, humidity in the air, smoke, who's listening, what you ate before singing, time of day,...etc. Trying to figure out what makes your voice work better may be easier than trying to figure out what messes it up, but the effects probably still aren't as pronounced as you may think.