The conclusion doesn't necessarily follow in the quoted question below. It could be the case that the total number of bicycle accidents have increased thus the number of heads injured have also increased despite helmets being effective. It should be worded "but the percentage of head injuries from bicycle accidents hasn't decreased" instead. Thank you.

"Over the past decade, there has been a marked increase in the use of helmets but the number of head injuries from bicycle accidents hasn't decreased. This suggests that wearing a helmet doesn't necessarily lower the risk of head injuries in bicycle accidents."

why is "the only" missing as a sufficient indicator in this table?