Martinique timing issue.

Ok, we have an 89 Martinique and my grandmother has 89 Martinique c/b. Well we set the timing on our boat last year to 10 degrees btdc. my grandmothers was set a couple years ago to 10 as well. Well a month ago i checked hers for the fun of it and found it running at 0 TDC. set it to 10 and running much better. WE put new plugs in our boat last year and set the timing. I checked it just today and found it 1 ATDC!!!!!! its running so much better now than it was earlier his morning when we did skiing and such.

Now both boats have ALL ORIGINAL ignition. Except for plugs. My question is how on earth did the timings get so far off???? When I undid the retaining bolt on each engine they were pretty tight. Both boats have the PCM 351W PowerPlus Package.

Oh our boat has 760 hours. (had 650 when we got it in fall 2010 and around 700 when we did plugs and timing last year) My grandmas only has 440 hours and doesn't see much use.

oh might want to add that we have had our Supra since 1994 and never done anything to the timing and checked it this year to find it running dead on after years of hard use and over 1500 hours.