SQL SUM giving unexpected results

I am getting some unexpected results when running a SUM on a SQL database. The database is used by a 3rd party program, and I've written my own program to extract some data.

The 3rd party program is a helpdesk system, and logs technician times against jobs. The technician actions are logged as either billable or non billable.

I need to add together the amount billable hours and non-billable hours for the current month on a per-technician basis.

Here's the problem. If I run two separate queries - one to get the billable hours and one to get the non-billable hours then in my program, add the results together, I get the answer I am expecting (in this instance 177.77).

SELECT SUM(timetaken) FROM dbo.ACTIONS where who='Tech1' and When >= '08/01/2012'

SELECT SUM(nonbilltime) FROM dbo.ACTIONS where who='Tech1' and When >= '08/01/2012'

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If I try to perform the addition in my SQL statment, my answer comes back as 174.27 and I don't know how to troubleshoot the problem...

SELECT SUM(timetaken + nonbilltime) FROM dbo.ACTIONS where who='Tech1' and When >= '08/01/2012'

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