The Si and EX models of the 92-95 civic came with the D16Z6 motor, a SOHC VTEC engine. The DX, LX, CX, and VX models had the D15B7, D15B7, D15B8, and D15Z1 motors, respectively.

My assumption is that Honda put a larger diameter exhaust pipe and muffler on the performance 1.6, rather than the economy 1.5s, and that accounts for your difference in part numbers.