Without something to work with this could be absolutely anything. There are at least few dozen or more possibilities. Many of those answers come from the ddl.

I would warn you VERY strongly to stop using NOLOCK like you are. It is bad enough in specific queries but in a function that appears to be dealing with money you are asking for serious problems. You need to understand what this hint does and the dangers it can bring.

Here just a few articles that explain this hint.

http://www.jasonstrate.com/2012/06/the-side-effect-of-nolock/[/url]

http://sqlblog.com/blogs/andrew_kelly/archive/2009/04/10/how-dirty-are-your-reads.aspx

http://blogs.msdn.com/b/sqlcat/archive/2007/02/01/previously-committed-rows-might-be-missed-if-nolock-hint-is-used.aspx%5B/url%5D

http://sqlblogcasts.com/blogs/tonyrogerson/archive/2006/11/10/1280.aspx

http://sqlmag.com/sql-server/quaere-verum-clustered-index-scans-part-iii