First, some normal gaps are to be expected and, in some instances, even practical. Floor boards will expand and contract with change in seasons and moisture levels. Unnecessary use of filler could result in buckling of the floor or other damage.
If you’re running into gaps during installation, the mill is not to blame. Wood is a natural product and variations in width do occur, even with Select-grade wood: A board might be a bit narrower on one end and wider at another. It may even be a bit warped. These deviations from the norm are typically more apparent in longer-length boards but the issue affects hardwood and engineered hardwood floors alike.
No matter which length or type of wood you’re working with, the fix is the same. If your visual inspection reveals that you’re working with boards that have slight variations, you’ll need to take a more thorough approach to installation. Instead of nailing a board in the middle, tighten the plank on one end against another board and nail it down. Then, continue to tighten and pull as you move down the board. For shorter boards, you may only need to do this twice (at each end). Wood fillers can be used to fill in uncontrollable gaps.
Avoiding gap issues during installation will require more labor and time upfront but the results are well worth the time and effort.