I'm laying a pine decking board with 140x32 premium pine decking boards. I'm using 3mm spacing, as they have been stored inside and seem dry.

Need to butt join them as boards are not long enough, which is fine. However, they are not EXACTLY the same widths. Some boards are 1400mm, some 1398, some 1402. So, it can be that I butt join a 1398 board with a 1400.

On the next board, can't use 3mm spacing everywhere I figured. So, using 3mm next to 1400 mm board, and 2mm extra (= 5mm) next to the 1398 board. That should keep it straight & square I thought.

However, after laying 4 lengths, when I measure the total width at each end, one side is 5mm longer? 

I need to lay 38 lengths in total so plenty of time to correct this, but why did this happen? Or can it be because one single board is not exactly same width everywhere?