Not sure what the question is about. ALL entries and exits must be declared in the residency calculation, regardless of the extent to which there is, or is not, some record of the trip. Best, most reliable record for an applicant, is that kept by the individual who independently records each trip abroad.
CIC, in contrast, may look to multiple sources to ascertain for itself how reliable the applicant's accounting of travel is, including the CBSA travel history but potentially other resources as well (depending on the level of inquiry CIC engages in for the individual applicant). While CIC's general purpose is to verify the declared travel, it is looking more for indicators of inconsistencies, omissions, or potential use of an alternative Travel Document . . . that is, looking for indications as to why the applicant's declarations might not be a reliable, accurate accounting of all travel abroad.
In some instances, an applicant's reported re-entry into Canada might not be verified in CIC's assessment. Whether or not that can be problematic depends more on a wide range of other factors, including especially other indicators of the applicant's veracity and reliability, including in particular, of course, the persuasiveness of the applicant's declarations as to place of abode and employment in that period following the declared but unverified re-entry.
Some of us, for example, have had dozens of trips which are not reflected in passport stamps (typical for visa-exempt passport holders traveling by land between Canada and the U.S. for example), and for which the CBSA travel history was not complete either, but we encountered no concerns about our residency calculation declarations. In contrast, an individual for whom CIC would expect there to be a passport stamp reflecting a declared trip might encounter a skeptical CIC if otherwise it appears to CIC that (1) either the declared entry did not happen, or at least not on that date, or (2) possibly the applicant had and was using an alternative Travel Document not disclosed.