If someone clicked an ad 60 days before buying, should the ad get credit? The attribution window decides. With a 30-day window, it doesn't; with a 90-day window, it does.
Windows are one of the main reasons tools disagree. Meta Ads Manager defaults to 7-day click and 1-day view; Google Ads click-through conversion windows default to 30 days and can be set up to 90. Your own attribution tool might use 90 days. Pick a window that matches your real sales cycle: short for impulse purchases, longer for considered and high-ticket purchases.