SUM

Hello, 

I find it odd that 

when I use the fomula: Total of Initial & Ext = SUM('DAC datasource'[Initial Total])+SUM('DAC datasource'[Ext Increase])

The total shows :

Then I take SUM off :  Total of Initial & Ext = 'DAC datasource'[Initial Total]+'DAC datasource'[Ext Increase]

It gave me correct sum : 

I would've thought both should return the same result or is it I misunderstood SUM function. Hope you could help me clarify.

Ps There is no filter applied so no hidden rows. 

Many thanks in advance,

Anna