To have a trend you have to be flying hours. If you are only doing 20 a year you have not got a trend just a snapshot. Our 0-200 gives 72/76 on all four every year based on about 100 hours flying. Except last year (see below).
As has been said diff comp testing doesn't always tell the full story and really it is a diagnostic tool.
Last year we had our 150 done. It had no reported loss of power yet on diff comp one cylinder was 8 over 80. Exhaust valve guide totally shot.
I did an R22 and got 0 over 80 on all four!!!!!!!! After checking the equipment and getting someone else to do the check we had the same result. All four exhaust guides shot to buggery and no reported loss of power!!!!!!!!!!!!!!!!!