Formula result changes once a cell is activated

[AntiPivotTable]

I am puzzled by this issue. I have two cells that have the same exact value. However, when I take the difference between the result is not zero. instead, it's 6 -- see below. The culprit is cell B3. The result changes (..to zero, as it should be) when I click on cell B3 and press enter.

I have no macros running and the cell formatting is the same in all cases (ie, they're all as 'Number').

Why can this be happening and how do I prevent it?

 

[mole999]

excel can deal with a maximum of 15 significant digits

 

[AntiPivotTable]

excel can deal with a maximum of 15 significant digits

Can you please elaborate? I see the same number of digits in both rows 3 and 4.

 

[mole999]

Just a snippet of information about this well known limit > http://excelunplugged.com/2014/05/19/15-digit-limit-in-excel/