Agreed - as long as you have separate rating systems: CFC Regular, CFC Active, FQE, USCF, FIDE etc. you will always get differences.

From a statistical point of view rating is simply an average performance estimate - and on any given day you never know which opponent is going to face you at the board. I have beaten people 500 pts higher than me and lost to 450 below me. "Having a good day" and "having a bad day" are concepts not reflected by the rating system. 25 years ago when I lived in Winnipeg I knew a number of players who ALWAYS played 200 points above their CFC rating when they went to Minneapolis twice a year. They figured it was due to their being out of their home environment and it being 'party time on the road'.

Again - on any given day one never knows which player you're going to face at the board and this is particularly true the further you get from master strength.

(I'm definitely not running for the job of ratings auditor but DO have the academic background to read the technical papers from the USCF and FIDE on rating systems and had some great chats with one of my former professors, the late Nathan Divinsky on this subject)