[amibroker] Sum Reference Not Same as Simply Adding Previous Data Points Manually

[davemabe2000]

I'm trying to add the volume of the three previous bars (not including the 
current one) in my backtest.

If I sum them up "manually" I get one value:

VolumeSumA = Volume[i-1] + Volume[i-2] + Volume[i-3];

But when I try using Sum and Ref to do it I get a different value for some 
reason.

VolumeSumB = Sum(Ref(Volume, -1), 3);

But VolumeSumA != VolumeSumB.

What should I be using for Sum and Ref to get it to work properly and equal 
VolumeSumA above?