This process uses the 'Quirky Update' without the essential requirements being in place. As such it is unreliable & will almost certainly give incorrect results in the future (or may have already done so).

Quirky Update is where variable & column are updated together:

update #runTotal

set @runningTotal = runningTotal = @runningtotal + FieldToSum

See this:

http://www.sqlservercentral.com/articles/T-SQL/68467/

for more details - the list of requirements for Quirky Update are towards the end.

It may seem a bit complicated - but unfortunately it is! 🙂