Three things:

Are they similarly equipped: both DL sedans with an automatic for instance
Do both have perfectly functioning brakes
What are the rear end ratios

The last one is the most likely suspect. 1987 automatic has (likely) a 3.73 but so does the 1992. In the US, the 1992 auto had no optional ratios but in 1987 there were options. In addition you have no idea (likely) what rear end ending up in the 1987 or 1992 unless you have owned it the majority or its life.

So if it has a 3.31 one from the manual in an automatic, you would have about a 12% increase in low speed acceleration. This is assuming the relationship is 100% linear which it is not...but you get the idea.

So when you time the 0-60 for both are we talking a difference of 1 second or more like 3-5 seconds? If we are talking 1-second that could be manufacturing tolerances, difference in tire diameter, more crap in the trunk, different in wind resistance from the windshield trim, etc. 3-5 points to significant difference in rear end, a dragging brake or a significant engine performance issue.