VBConversions superior accuracy is due to two things: superior architecture and massive continual testing.
Rather than rely on free lexical analyzer and parser tools, often written in non-.Net languages, VBConversions hand writes all code for maximum flexibility, and strives to fully understand your code, including digging through referenced assemblies to resolve identifiers. This knowledge is critical in accurately translating VB.Net, with its many ambiguities, into C#. Finally, VBConversions will compile your converted C# code and then fix it by parsing and understanding the compiler error messages, in much the same way a human would.
In addition, VBConversions relies on massive testing, and has spent the last 15 years amassing a test bed of over 15,000 VB.Net projects. Such continual regression testing is absolutely critical to achieving and maintaining high accuracy. New projects are continually added to the test bed, and each release must pass a full regression test before it is released to the public.