Referring to analysis by Robert Engen present here, this claim was based on the numbers in the books "On Killing: The Psychological Cost of Learning to Kill in War and Society" by David Grossman; and "Men against Fire", by SLA Marshall. However, there is no research/statistical evidence to support the claimed numbers.
Here, Marshall becomes extremely problematic as a source. Historians and researchers since the 1980s have been consistently demonstrating that Marshall did not have the evidence to back up his claims. Roger Spiller, among the first historians to publicly criticize Marshall, claimed that his ratio of fire numbers were “...an invention,” and that “Marshall had no use for polite equivocations of scholarly discourse. His way of proving doubtful propositions was to state them forcefully. Righteousness was always more important for Marshall than evidence.” Other historians discovered that none of Marshall’s aides and assistants could ever remember Marshall asking the troops questions during the group interviews that had anything to do with whether they had fired their weapons.
In the surviving field notebooks used by Marshall during his interviews, historians have found no signs of the statistical compilations that would have been necessary to deduce a ratio as precise as that found in Men Against Fire. Such a precise, surprising number as the 15 to 25 percent ratio should have required a great amount of hard work and data-gathering to arrive at, but there is no evidence that Marshall carried out the statistical legwork his claims imply. The only interview notes actually located were found in an archive of a Maryland National Guard division, wherein soldiers testified to having used their weapons in action. There was no mention of the ratio of fire. Source: KILLING FOR THEIR COUNTRY: A NEW LOOK AT “KILLOLOGY”
Also the actual numbers based on Canadian armed forces showed them firing too much, wasting ammunition, and giving away their positions as detailed here.
The phenomenon of “too much fire” is really a corollary to the effectiveness of
the weapons being used, because as we have seen, Canadian soldiers relied heavily upon infantry firepower in battle.