W-L Ratio alone is not a good measure... it includes all spars, not just recent spars; and it does not incorporate your opponent's skill.
If your W-L is 2-1, and you win a spar (3-1), your ratio increased 50%
If your W-L is 2000-1000, and you win a spar (2001-1000), your ratio increased 0.05%
Law of Large Numbers. More spars = less ratio change. Point being, its very difficult raise your ratio significantly the more you spar.