Adding Tagtot

Hello,

I currently have an equation where I totalize the tonnage from a conveyor for the previous day and the equation is written as:
tagtot('TAG', '-1d','*')*24

The tag has units of tph which is where the 24 stems from. This calculation is performed once a day at 6 AM.

I'm looking to split this calculation into two parts, one for dayshift and one for nightshift. 
I wrote the tags as:
D/S : tagtot('TAG', 't-24h', 't-12h')*12 
N/S : tagtot('TAG', 't-12h', '*')*12

These are calculated at 6AM once per day. 

Both equations above are valid... but when I sum them together I get a different result (sometimes higher, sometimes lower) than the actual first equation. Could someone please indicate why the two are not adding up?

Thanks!