Sesame supports two RDF parsers: HP's ARP parser, and Sesame's own parser (which is so new that it doesn't even have a name yet). The ARP parser currently is the most standards compliant in that it checks almost everyting for compliance with the RDF specification. Sesame's RDF parser is by far the faster parser, but it doesn't check everything yet (e.g. it doesn't check whether URIs contain characters that they aren't allowed to contain).
The default parser is still ARP, as we would like to have some more feedback on the compliance of our own parser to the specs. You can switch parsers by changing the value of the rdfParser parameter in the web.xml file. Supported values are ARP and Sesame.