Lolking uses end of game screen statistics.
So, if at the end of the game, you still have a Boots of Speed in your inventory, you're probably on the losing team because you never got enough gold to upgrade it.
This is also why every Tier 2 / Tier 3 item has a higher than 50% win rate for the most part. If you have one of those - that's a good indication of a healthy gold flow. Look for outliers if you want to draw reasonable conclusions without having to run regressions.