Solaris SONAME not matching libraries

I have noticed some odd behaviour under Solaris when various libraries are installed from the source package. Using 8.2.4 tarballs on Solaris 8, Sun Studio 11 (cc: Sun C 5.8 Patch 121016-05 2007/01/10), I get the following behaviour (using plpgsql.so as an example, there are many others) :

# elfdump -d plpgsql.so | grep SONAME                                                                                                                            

       [2]  SONAME           0x10da            libplpgsql.so.1

As you can see, the file is installed with the name "plpgsql.so", although it's SONAME according to elfdump is set to libplpgsql.so.1. 

Why the discrepancy ? 

Many thanks in advance for any pointers, advice etc.

-Mark