Interesting question - it caused me to perform a bit of research.
The integration of "East" and "West" did not occur immediately in 1990 - the economic and financial aspects required years and cost W. Germany dearly, but it did boost W. Germany's economy in the near-term.
E. Germany's economy was mostly state-owned, and the W. Germans privatized it after the reunification.
Even before economic unification, the West German government had decided that one of its first tasks was to privatize the East German economy. For this reason, it had taken over in June the Treuhandanstalt (Trust Agency, commonly known as Treuhand), which had been established by the GDR to take over East German firms and to turn them over to new management through privatization. The agency assumed the assets and liabilities of about 8,000 East German enterprises in order to sell them to German and other bidders. By the time the Treuhand was disbanded at the end of 1994, it had privatized some 14,000 enterprises.
As economic unification proceeded, issues that had been recognized but inadequately understood in advance began to surface. There was massive confusion about property rights. As wave after wave of Nazi, Soviet, and later GDR expropriations had taken place between 1933 and 1989, there was often little knowledge of the actual ownership of property. More than 2 million claims on properties in the territory of the former GDR were filed by the December 31, 1992, deadline. As more claimants emerged, with many winning cases in the courts, potential investors were often scared off.
Another problem was that East German production costs had been very high. The conversion rates of East German marks to deutsche marks often kept those costs high, as did the early wage negotiations, which resulted in wages far above the productivity level. Western German firms found it easier and cheaper to serve their new eastern German markets by expanding production in western facilities.
A third problem was that the inadequate infrastructure also became a problem for many potential investors. Telephone service was improved only very slowly. Many investors also complained about energy shortages, as many East German power stations were shut down for safety and other reasons. Roads and railroads had to be virtually rebuilt because they had been so badly maintained...
The dramatic improvement in the western German figures resulted from the opening in eastern Germany of a large new market of 16 million persons and the simultaneous availability of many new workers from eastern Germany. Many easterners did not want the shoddy goods produced at home, preferring western consumer products and food. Moreover, many easterners were coming to the west to work. By the end of 1990, as many as 250,000 were commuting to work in the west, and that number was estimated to have grown to 350,000 or even 400,000 by the middle of 1991.
http://www.germanculture.com.ua/library/facts/bl_reunification_aftermath.htm
As the bolded sentence suggests, possibly Sig did not want the "stigma" of an E. German-manufactered product. Perhaps they delayed omitting the "West Germany" until the reunification was more complete.
The Germans mights have differentiated for years after the reunification between E. and W. German products, because the cross-over wasn't so apparent.
I wonder what Sig has to say about this.
Monty