There is a difference between knots, however. Obviously some are larger than others. There are live knots and dead knots. A live knot is the knot of a branch that was alive when the tree was harvested. A dead knot is from a branch that was dead.
Live knots are much stronger.
Inside many birch trees is dark heartwood. This wood is contrasted with the white wood on the outside. Heartwood isn't necessarily weaker than the outside wood, but the water content is far less. If a sled member has a little heartwood and the rest is white wood, it will tend to warp strongly towards the white wood when it dries, shrinking more on that side than the heartwood side. When women peel birch trees to make birch baskets, the tree isn't killed.